If you can't find the presets in GarageBand as in the Standalone application, it's due to the fact that GarageBand runs in a sandbox.
For users of MacOS 10.15 and previous versions:
To move the presets of Standalone application to GarageBand, please copy Documents/PositiveGrid/BIAS_FX2 folder, and paste it in the following directory:
*Please click on the 'Go' menu in Finder while holding down [Alt] to access the'username/Library' folder.*
For users of MacOS 11 Big Sur and up:
Move Documents/PositiveGrid folder directly into Documents/GarageBand folder.