Like Matie said, push the box and then you can grab the gun.
If you need you can check (but careful to not get spoiled all the levels 😜)
Viewing post in Pascal and the Scale Gun jam comments
Well, that makes a lot more sense now! I really enjoyed my second playthrough of that! Inventive level design. I think I've seen a similar mechanic to that somewhere before, but I can't remember where.
I found the speedy controls a little too fast for my aging reflexes, but I can't argue that it was fun and a full complete polished game. Congratulations!