Well, I've made some progress. I now have to find a hydro-pneumatic accumulator for the gate, as well as a control valve, and based on some reading I did I also apparently need to use the drop from the mushroom boss to make some sort of grenades to remove the fungus on the gate. These are the 3 things I'm stuck on. Honestly, it's a bit disappointing how frustrating the end of the game is, especially if your build isn't too overpowered (and I wasted a lot of supplies taking out the Faceless base too - I'd say it wasn't worth it, but at least I got a lot of satisfaction from that) and by now, I'd just like to know where I must go to get these 3 plot items so I can move on with the plot. Could anyone help me out here once again? Thank you!
(Edit: Nevermind the valve, I've found that. Damn, that was some creepy and sad stuff in the place where it was held. Now, two more things - accumulator and where to make the grenades.
Edit 2: Got the accumulator too, damn heavy thing. I think I passed that building by 3 times already, thinking it was another one I had already explored. One more to go.)