I wanted to get around to doing this yesterday but I didn't
There's some notable issues with the pack that I feel I should point out. Normal people call this a "Criticism," I think? If I get something mixed up at any point, please let me know!
As a starting point, some general errors:
-Why are the map codes GUMxx? It doesn't seem to relate to much.
-The rotation of the maps goes to GUM04 before GUM03. I doubt this was intentional, but if it was, whoops!
-While a file size of 22.3 MB won't exactly kill anyone's family, it's a little large for just four maps. It's not horrible, though. I'd check the compression on the music files.
-Most maps have a massive overabundance of weapons. I'll get into it more in-depth as I go along, but just keep in mind that this is a problem that persists throughout the maps.
Now then. Let's address GUM01.
(http://i.imgur.com/xVRCUtd.png?1)
Sheesh, this is rather off-putting. The skybox isn't animated when I assume it should be, the textures stacked on top of each other look awkward, and there's that green. More on the green later. There's also that weird bend in the skybox? I'd recommend fixing that.
(http://i.imgur.com/2CGnEsP.png?1)
This room is, and I say this as respectfully as I can, pure eyebleed. The garbage textures and the fire textures manage to clash in amazing ways, and that green is back. I suspect that green is there because you used a texture meant for rails on a non-rail surface. It's a bit like using a picket fence texture on a wall. I'd recommend replacing the fire on the floor with a lava texture, but if you want to stick with the dustman texture set, then you may be hard pressed to find a suitable texture.
(http://i.imgur.com/T2goIvY.png?1)
There sure is a lot of Thunder Bolt here. While I strongly recommend against replacing the Thunder Bolts with other weapons, keep in mind that having more than one of the same weapon in a given room, especially used like this, is usually unnecessary. There are exceptions, but just keep that in mind.
(http://i.imgur.com/TsKgSUy.png?1)
This hallway really isn't necessary. Neither is the teleporter, now that I think about it. But yeah, you have a hallway leading back into the room you came from. It's not really needed, and the teleporter isn't really warranted on a relatively flat map of this size.
(http://i.imgur.com/0fnvon2.png)
(For those curious, the past four screenshots had their resolutions edited manually. I changed the in-game resolution just before this screenshot.) This hallway would honestly be really cool, if not for the fact that it's so claustrophobic. I'd recommend expanding this hall.
-There's other miscellaneous texture issues that can be seen in a few of these. Mostly, the blue and red texture is used inappropriately most of the time, and looks fairly jarring.
-The boss music starts when one frag is remaining, and there is no victory music. The boss music should start with five frags remaining.
-There's a metric assload of weapons. You've got stuff like Crash Bomber, Atomic Fire, Homing Sniper, and a whole slew of other things. Props for adding electric weapons to counteract Junk Shield, but I don't think Lightning Bolt was a good choice for such a confined map. Or it's a fantastic choice, either/or. It's a notable choice, though.
It's all uphill from GUM01. Let's see what GUM02 has to offer.
(http://i.imgur.com/8y89YL9.png)
There's a lot of issues with this skybox. For starters, you can see that the texture is cut off in places. In addition, some of the sides of the skybox are darker than the others. Another notable issue is that the sky on the "wall" part of the skybox is inconsistent with the "sky" part of the skybox, making the hilly background look more like a wall. Lastly, that skybox wall is reeaaaaaaaaaaally far away from the map itself.
(http://i.imgur.com/LjdlLFB.png)
This ledge is not impassable...
(http://i.imgur.com/jYrOIke.png)
...Which I use to get behind the invisible wall on the fence here. Also, why is there a fence just before a wall? Seems odd, but that's just an aesthetic qualm.
(http://i.imgur.com/TLpefD8.png)
These trees are incredibly blocky. I'm not exactly amazing in aesthetics, myself, but I would recommend trying to make these look more like trees.
(http://i.imgur.com/bK5Inbo.png)
While initially annoying due to its relative secrecy, I have to admit that this teleporter's location is pretty cool.
(http://i.imgur.com/G1c7Xms.png)
Holy crap, this room has a lot of weapons. It's also a bad idea to make the only entrances and exits to a room teleporters.
(http://i.imgur.com/sUHfzFE.png)
And to top it all off, you get a free ring boomerang upon leaving the room. That's odd.
-No victory music, again. The boss music plays at the correct time, though.
-I never counted the exact amount, but I think this map has the most weapons out of the four. It's either this one, or GUM03. Regardless, it's excessive and unnecessary.
That map was a notable step up from GUM01, so let's see if you improved even further in GUM03!
(http://i.imgur.com/IfBcQKX.png)
I presume this is the "Main room," as it's the center one. This map is structured pretty much like a giant W (or an M, depending on how you view it.) This creates the obvious issue of three separate dead ends, with the center room itself being one. An easy solution is to add a hallway connecting the three rooms on the side where none exists, but this would probably cause problems with the map's detail. Perhaps you could play with height and make tunnels or overpasses? Regardless, there's also a lot of tokens in this main room. Also, those conveyor belts don't move. Is that intentional?
(http://i.imgur.com/09qUiTk.png)
What's this thing in the center? Its textures are oddly mix-matched, and it obstructs most of the gameplay in the room. There's also a bunch of tokens in here.
(http://i.imgur.com/9rNVaNi.png)
This is a really narrow hallway. I like that you made the effort to have SOMETHING linking the two points of the W together, but a teleporter isn't usually the best way to create flow in a map.
(http://i.imgur.com/nALpgxB.png)
For minimal effort, somebody gets all this stuff. I'd recommend just removing this back section.
(http://i.imgur.com/vIah09t.png)
The other side's dead end. Like the other two rooms, this one has a lot of weapons. The two charge kicks and two quick boomerangs (the latter of which are not visible in this screenshot) being so close to each other is entirely unnecessary.
(http://i.imgur.com/DLGTPtf.png)
This skybox just looks weird. It shares the same inconsistency between wall and sky as GUM02. In addition, what is this thing directly outside the window? I'm honestly having a tough time telling.
-Like the other two maps, this one has a TON of weapons. I can't tell if this one has more tokens than GUM02, but the addition of Eddie increases the scope tenfold.
-The music doesn't quite loop properly.
-There's no victory music again.
And last, but not least, GUM04.
(http://i.imgur.com/csuQHDr.png)
This is... odd???? Is this intentional?
(http://i.imgur.com/j8rzQIy.png)
There's a lot of things to go over here. For one, this green filter is weird and off-putting. On that note, the filter does not exist under the 3D floors. Also, I chose this angle for two reasons. The first reason is to show off that discolored wall in the back there. The second is to show that Arrow Buster, Laser Buster, and Duo Fist are all in the same room. Hell, that's three different buster upgrades visible in the same screenshot! The problem should be evident. Aside from the filter, though, this room actually looks really cool!
(http://i.imgur.com/QsjQORI.png)
I might be stupid, but I legitimately thought you could walk through those windows.
-This map is significantly better about its weapons than the other three, but there's still quite a bit here. There's also a fair deal of redundancy. For example, Rush Coil and Tornado Hold are both here.
-There's no victory music here, either.
So, yeah. I'm no professional critic, so I likely missed a few things. I also didn't mention a fair deal of odd texture choices due to my relative ineptitude in aesthetics, myself. I hope this helps, though!