Really late answer but the problem continues to be relevant (I hit it trying to bring an existing git repo into p4 in order to practice migrating it back, haha). git p4 is intended ONLY for bringing git-side changes in a repo that was originally imported from p4. You can see this in git-p4.py, where the git branch to rebase or submit will be scanned by findUpstreamBranchPoint() to find the last commit that came from p4. This is done by looking for a log message containing text of the form "[git-p4: depot-paths = "//depot/path/": change = 3]". It keeps going backward until it finds the log or gets the error for going beyond the branch start. There is another tool gitp4transfer (untested) that can do this. Or you can work around by rebasing the foreign git repo branch onto one imported from p4 – see https://stackoverflow.com/a/29496432/10532990 – and then git p4 submit will work.