The GH5 is not able to sync TC with external devices. This Camera has no interface for TC. Only the GH5s has the ability to send and receive TC from a generator. The TC implementation in the GH5s is so poor, you can not work with it. Instead of File TC use Audio TC with the Gh5.
isn’t it better to ask this question to Panasonic or a website or forum of GH5 users?