This mod is NOT a normal mod. It does NOT go in the Mods folder. It is a fork of Mod the Gungeon, i.e. it is a patched version of the base Mod the Gungeon code with performance improvements.
Installation requires the base Mod the Gungeon installer.
Download GGB_ETGMOD_1.1.2.zip from link in modworkshop.
Start MTG installer.
Go to Advanced tab.
Drag and drop the downloaded GGB_ETGMOD_1.1.2.zip onto the Advanced tab. It should show the zip selected in a box under Advanced.
Run the installation.
Verify that version installed shows 0.3.3-ggb+1.1.2. This appears in the installer and on the game title screen.