Twitch sent you back here with an authorization code. Copy it into the Gerry launcher.
Authorization code
Switch back to the Gerry launcher window. Paste this code into the Twitch auth dialog and hit OK.
You can close this tab afterwards.
No ?code=... parameter was found in the URL. Authorization may have been cancelled or this page was opened directly. Re-run the Twitch auth flow from the Gerry launcher.