It can't be opened using gms2, gms2 uses .yyp files and this has a .gmx file and will just work for 1.x versions.
RickyG's UNDERTALE Engine
A downloadable tool for Windows, macOS, and Linux
This is the UNDERTALE Engine made by RickyG (the ABSOLUTE GOD of fangames), which was taken down, and left those who had it as the only ones who would make it anywhere. This one, however, is a reupload of a reupload (The MODDED UNDERTALE ENGINE by novaquantum). And the creator said that it was broken. I was able to fix it by manually importing them all to a new project (which took hours, btw). Feel free to use this on any UNDERTALE fangame (but PLEASE include RickyG in your credits, ok?).
How to use:
- Unzip the folder
- Run inside GMS 1.x (idk if it runs in gms 2...)