Wow. This. Is. Amazing. It looks brilliant, control is mostly nice and snappy (sometimes you keep a lot of momentum when attaching to a new hook, which can be especially problematic in the second green hook section where have to get past to vertical outcrops of spikes). The gun feels really powerful, even without any sound, great game feel overall.
How did you get the light to be pixel scale, is it the pixel perfect render mode of the URP?
One thing missing are sounds. A nice railgun type sound for the gun and an attaching sound (maybe BFXR), maybe some winding when adjusting the rope length, would have gone a long way for relatively little effort. And maybe enemy sounds. By the way: The fight music seems to trigger only when you are in a certain area, maybe you should do that based on enemies nearby, or, once you get to implement that, when an enemy is aware of you.
I was a little confused as the end, because the coming to steam... text only appeared a long while after I attached to the last hook and noticed there is no further path. You could have been a little more liberal with the checkpoints towards the end, I explored some deadly pits out of curiosity, and had to do the whole green hook section again, multiple times over.
I jumped into the game unprepared, so I was confused at first. I thought you could attach a grappling hook with your gun, but that did not work. Maybe you could add a tiny tutorial at the start (maybe A/D, attach to hook, W/S, space, how to fire gun), but figuring out the rest was extremely rewarding.
All in all, excellent work, especially for a 72-hour marathon!