If you host your TYPO3 extension on Forge using Git, you need to change URL for your remote. Here is the information from the server team: link.
One additional note: if you need a write access to the repository, you will need a second command.
Direct write access:
git remote set-url --push origin ssh://email@example.com:29418/TYPO3CMS/Extensions/yourextkey.git
For Gerrit push:
git remote set-url --push origin ssh://review.typo3.org/TYPO3CMS/Extensions/yourextkey