Bepinject is a utility for Gorilla Tag mod developers that allows them to use Zenject framework. In other words, if you have a mod that was built using this framework, the mod will not work and you have to install this library.
Bepinject can be automatically installed using Monke Mod Manager (look for ‘Libraries‘ section in the menu) along with other libraries:
Alternatively, download both files below and unzip them to BepInEx/Plugins folder. You have to have BepInEx installed beforehand (or, again, just use Monke Mod Manager, it handles all the hassle).