G-SPET now works with GameMaker Studio 2 projects that were created in (or converted to) GameMaker version 2.3 or later.

GMS2 version 2.3 brought big changes , including an overhaul of the way projects were stored and organized in the file system.
This update for G-SPET (v1.1.00) allows it to work with the new project structure. It still functions exactly as it is did before, but now it  works with  all GMS2 projects again (which still includes older, pre-2.3 projects, as well as the new, post-2.3 projects). 

It seems unlikely that YoYo would do another big overhaul on the project files, so hopefully this version of G-SPET will continue to work for a long time.

How to update:

Download the attached zip folder from this post, or from G-SPET's itch page.
Extract the folder inside.
Delete your old G-SPET folder and replace it with the new one.
Then just run G-SPET.exe to use. 
(You can create a desktop shortcut to the exe for convenience, but don't remove it from the directory).