I'm trying to set every weapon in the game to be fully automatic, how would I go about doing that? I've downloaded the GML code editor and was wondering how to actually edit the weapon objects.

If you really just want to make every built-in weapon automatic, unconditionally, you could do

for (var i = 1; i < 128; i++) {
    weapon_set_auto(i, true);
}

Replicating Steroids-like behaviour is a little trickier and would probably involve timely setting clicked variable for the player.