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 macOS 10.15 and previous versions:
To move the presets from BIAS FX 2 Standalone to BIAS FX 2 AU in GarageBand, please copy "BIAS_FX2" folder in /Documents/PositiveGrid/, and paste it in the following directory:
*You have to locate the ~/Library folder by the following instructions:
1). Launch Finder.
2). Click "Shift + Command + G" then copy & paste "~/Library/Containers/com.apple.garageband10/Data/Documents/PositiveGrid/BIAS_FX2" to the "Go to the folder" window.
For macOS 11 Big Sur and up:
Copy & paste the /Documents/PositiveGrid folder directly into /Documents/GarageBand folder.