http://localhost:8191 (as included in the instructions), or if you're using Docker, then it'd be whatever your local device IP is, or the container IP or name, followed by the port.

Okay, I typed set LOG_LEVEL=debug in the Windows command line and ran flaresolverr.exe there.

Then I specified my API address in the FlareSolverr API URL field, but it still didn't work and GTorrent didn't load:
An error occurred while testing this indexer
Exception (gtorrentpro): FlareSolverr was unable to process the request, please check FlareSolverr logs. Message: Error: Error solving the challenge. Timeout after 55.0 seconds.: The operation was canceled.
Click here to open an issue on GitHub for FlareSolverr.

Docker:

• 21hsmw/flaresolverr:nodriver
• alexfozor/flaresolverr:pr-1300-experimental

OR

Run from source:

• https://github.com/21hsmw/FlareSolverr
• https://github.com/AlexFozor/FlareSolverr/tree/drission-page-experimental

Sorry for the long absence, but I changed the "FlareSolverr API URL" field to http://localhost:8191, and Jackett (for now) found the GTorrent site.