All you have to do is follow the G-man ending until you get to the part where he offers you to wipe the possibility of our boi getting cloned, this puts him in that one specific room on the map where the slug comes out of the wall in a box and you're supposed to kick it open.

Now do the following
1. Cheats on
2. Godmode, Impulse 101
3. Kick G-Man's dumb ♥♥♥♥♥♥♥ face in

You get teleported to a battle that you're supposed to lose, and since you win- first things first

"YOU CAN'T KILL ME I HAVE GODMODE"

Second thing, you need Noclip now and you get out of that map into the great abyss with a few floating things in the distance, you'll notice that of course they are the others rooms from the ending including the two black chambers where Gman speaks to you, the room with the slug you're supposed to kill to wipe your memories / stop the clones and of course the apartment.

You will now travel back to the room with the slug and see G man just chilling there, every time you do ANY damage to him, he will teleport you to the Black room before the alien fight, but since you have Noclip you just fly out of it and back to the room with G man. The easiest way to kill him is to throw all the SLAMs you got with the impulse 101 cheat.

Upon wiping him, Walker is gonna repeat the "You can't kill me" line and voila, He's actually ♥♥♥♥♥♥♥ dead and you can kick his ragdoll around or whatever.

AND

Once youre done with all that, you can use the Noclip to find the apartment part of the ending and just enter the trigger that leads to the credits.

nothing actually happens besides the dark screen, probably because by following the guide you've bonked the scripting of this game beyond repair, BUT IT WAS COOL THOUGH RIGHT?

Thanks for coming to my TED talk


Also the same steps can be done if you kick him in the apartment, its just that the location of where Gman stays depends on where you kicked him. And using the Xen grenade on both the alive and dead g-man results in a crash.

Have fun