I got stuck at the exact same situation as y'all and I got it fixed. Since I got stuck in the process for the self-hosted Gitlab instance I created a new application for my organization whose source-code I put on Github. That could be any example app or whatever. It doesn't matter. You just have to have a different app that you can use to finish the setup once on a different platform. You can delete it later in AppStore Connect. Once this new workflow for the "Decoy App" is completely set up you are able to FINALLY get rid of the stuck Gitlab integration under the Users & Access tab in the App Store Connect backend. Once deleted you can start the Gitlab integration from the beginning and that worked for me!