... No, DECORATE is in GzDoom; GzDoom is based off of zDoom. DECORATE is not in Legacy.
Legacy is a strange engine that isn't really related to them except that it's based on Doom and uses some Boom extensions. Because it lacks any way to define new actors, you need to use DEHACKED, which basically just allows mod authors to RE-ARRANGE frames (you can't create new frames) and re-arrange Vanilla Doom actions.
C: I tried to install GZDooM on here, it says to put my WADs in the GZDooM directory, at Home/username/root/gzdoom
B: My computer is probably not powerful enough.
I've gotten permission from Strife for me to port his level pack's to GZDoom. More info soon.