I just tried out RaceChrono a couple of weeks ago and also recorded some video on my Hero 6 Black. I signed up for the Pro trial and tried to import the video from the GoPro. I can get it synced up to the start/finish line, and fine tune it with the +/- 50ms buttons in the first couple of corners so that it matches the accel/decel curve, but by the time it gets to the end of the lap, the video is always a couple of seconds ahead of the session data recorded by RaceChrono.
I've tried both the original GoPro MP4 files and my post-processed combined video file with the same results either way. Am I doing something wrong here? The video is 1080p60 and my phone is a Pixel 1. This doesn't seem like a slow video playback issue, since the video is faster than the session data.