After you finish the club you have to go back to the start, upgrade your weapon and get ammo once you get near the starting area again. Go back down like you did at the start and the red lift on the far LEFT works now. Go down use the colored levers to move the same colored container. You will have to go back and forth a lot. Red container should be the last one. Once you get to the horrible run and gun while Gigante chases you, that will be the last bit till the end. You escape for now... That is your reward for getting to the end.
The utter BULLSHIT that happens while trying to run and gun, might not be worth it to you. I fully upgraded the sub machine gun, and wasted Gigante a few times, kept respawning or coming back to life, so it seems you have to escape. I just killed the flower spore (tentacle plant?) that will hit you 75% of the time you run past it and the regular crab monster that you can't run past. You can run past all the flys and wurms, if you are fast enough. Make sure you hit E at the door and get to the lift as soon as you can. No use having 1-20 flys stun lock you at the end. Getting hit once why running and aiming seems to bug your gun to not shoot?