How can i launch gzdoom from gog galaxy ?
i tried to add gzdoom manually to gog galaxy
but i didn't found such option in the add games manually in the menu.

Sorry. This is late, but I oughta say how i made this work.

I did this for the ZDL mod launcher, but imagine it'll work for gzdoom too.

GOG Galaxy->Manage Installation->Configure->Features->"duplicate the default executable"->"Change the used exe to your gzdoom or whatever source port or mod launcher you wanna use"->"launch game with that shortcut now in GOG galaxy"