Getting an access violation error on trying to load this mod up with Strife. Version of GZDoom is 4.5.0 (currently latest stable release), used with ZDL 3-1.1 (using ye olde drag and drop instead of having ZDL around as a mod loader had no effect on results), no other PWADs added. Guncaster version is latest linked on the OP, 3.888
Given this is with the new PC I'm not sure if it's an "I'm missing something that my old PC had that's missing in the new one for some insane reason" issue or a recent issue. I'd need to hook up the old PC again to check whether the version of Guncaster I have on there is an older one.
Unrelated, this error screen now makes me tempted to see what happens if you load Guncaster up with that Touhou Doom mod, to see if there's any funny tweaks to the dialogue like there is when it's loaded with Strife.