For custom weapons, the ID is a string with weapon file name, so "some_wep.wep.gml" -> "some_wep". Also see if the problem occurs in the beta version if you aren't on that yet, I think there was some issue in 9921.
Still says GML Mod can't find the file, I'm in 9921 so I'm assuming that I'll have to go a couple updates back?
Forward, not backward. Beta builds are on Discord (see link in itch description) and the current beta is 9935
