Is therere no way to use my own VST library/3rd party plugins in Groove Agent inserts?

Virtual instruments Groove Agent
1

As the title asks

2

You load the inserts e.g. into the instrument track that is created when you load GA. If you load GA as a rack instrument, there is a folder track called Groove Agent and in there you load the inserts into the Master track. This is the same for all VST instruments.