Hey! I like playing games and saying what I think about them. I realized I maybe shouldn't be yelling public criticisms on twitter though (as was my habit for a month or two), so I've put to words some critical, opinionated feedback on your game as a player here in semi-private. Feel free to hide or delete if and/or when this is not the kind of stuff you want floating around your community! I won't mind at all. But I thought you might find this useful.
So I'll put the good stuff first and then the bad stuff after.
But honestly it'll mostly be bad stuff, not because your game is bad, but because I'm picky as hell and usually even when I'm trying to play something to enjoy myself all I can do is think about it as a design project :P
Oh, and... I beat the game, but that's it. I got the first achievement. More on that in 'bad stuff'.
Good stuff!
The game is short, sweet, quick. Falling so quickly to the next room is still a bit disorienting when I'm trying to catch my shell from the previous room and the invincibility period is also a bit disorienting but I'm a big fan of how it cuts to the action so fast. No time wasted.
Related to the above: the enemy design is pretty basic, but it's good! The worlds are short and don't overstay their welcome. The enemies within each world differentiate it from the previous world.
The basic action of shooting an enemy and catching the bullet which I used to shoot that enemy is pretty satisfying when I do it right, especially when shooting enemies from above and the shell jumping up into me while I fall into it. That interaction feels great. I'm also a fan of how enemies interact with the bullets (the bullets get floated up by them, kind of this interesting unpredictable element to add to catching the shell).
I liked the 'use mouse' button on the 4th(?) floor. Cute.
Bad stuff!
Holding the jump button to shoot down is a little bit... awkward. If I press left and shoot, I get immediate feedback despite the animation, which is good, it's great, it's what I was hoping for. If I hold jump and shoot, the timing makes what happens kind of ambiguous in a way that is inconsistent with the way the left/right arrow keys interact with shoot direction.
Perhaps I wasn't in love enough with playing the game to get more achievements, but it felt very unclear how to "really" beat the game. Possibly attempting the other achievements would have gotten me there? I played a couple more times and then quit at this point because I didn't really feel like I had a direction to go in. YMMV as other players more interested in / willing to spend time digging around may not give up as easily as I did. I'm an impatient fella.
EDIT:: This possibly belongs in 'personal-taste' section. I never got the impression I was in a whale's gut. I pay a lot more attention to systems than to visuals and to top it off I played in a well-lit area so probably some of the darker details got missed. There were no gut-like enemies, no gut-like mechanics. there were elevators and rooms and I was a guy with a hat and a gun. If contrasted against a setting that genuinely communicated "I'm in a whale's gut" I think some of these elements could be funny, or cool, or weird -- but there's too many incongruous elements. Instead of feeling like "I'm in a whale's gut and there's elevators down here, how weird", the gutwhale part really plays second fiddle and I pretty much stopped thinking about it completely, past my first run. (Which may be fine for your purposes! I didn't really worry about it until I thought back and remembered, huh, right, this game was called gutwhale...)
Your combo system doesn't communicate itself very well. It's hard to tell where I'm at in my combo (am I at 20x or did I just screw up so this next shell doesn't actually matter?).
The "shell catching" activity required by the combo system feels... curmudgeonly to me. Very picky. If I miss an enemy, the punishment is pretty severe. Enemies that take multiple hits make catching the shell super hard, whereas with most enemies it's very easy. Clusters of enemies make it difficult. I guess what I'm saying is the risk/reward matrix feels really inconsistent and not in a joyous and exciting way, just a kinda sticky, annoying way.
Some personal-taste bad stuff follows.
OK, now take all my feedback about your combo system and the "shell catching" with a grain of salt because: I'm not a fan of combo systems, at all, and I never have been. That style of perfectionist don't-break-your-combo playing just straight up clashes with my desire to screw around and play freely within an interesting system. Trying to catch a shell never feels like something I want to play with, and only ever feels like something that I have to do. I found it restrictive, but I almost always find combo systems restrictive. So I'm probably just the wrong type of player.
I think the roguelike/roguelite structure is extremely overplayed, and a very designer-trap pattern to fall into. In my underdeveloped opinion: It's really fun to design something that generates new levels for you every time because you kinda get the experience of being a player seeing new stuff every time, but as a player that's... the norm. Often I see the value of procedurally generated environments as "replayability" but if you haven't designed a procedural engine that can compete with the genuine craft and novelty of hand-designed levels, you're basically burning up your playability for replayability, which seems backwards to me. But as with above, take with a grain of salt. This my just be a personal chip on my shoulder -- as a designer and player I'm personally not a fan of the roguelite pattern.
EDIT:: Ignore my silly roguelike rant!
The soggy bottom bun of this compliment sandwich.
You made a game in a month! Congrats! I just wanted to end on a good note. Like I said above I'm not really a combo-lover or a rogue-lite-lover, so it's difficult for me to say what's great or what's not, but I hope you can find the people who will absolutely adore your game. I apologize for not having more good stuff to say; genuine positive feedback about what someone loves is so much more important than posts like mine :p