Keep both originals. GPX is convenient for interchange, but FIT may contain fields that a basic GPX export does not preserve. I would archive the original activity file and make a GPX copy only for tools that require it.

For sync, do not start by guessing the clock offset. Pick a distinctive event that exists in both timelines: a stop followed by hard acceleration, a sharp turn, a jump, or another moment you can see in the video and identify in the speed trace. One matching point gives you an offset. A second point later in the clip tells you whether there is real drift.

Also decide whether you want one long activity mapped across several camera clips or a separate telemetry segment for each clip. For a ride with many starts and stops, I prefer to keep the activity continuous and align each video clip against the same master timeline.

That makes it easier to diagnose gaps. If the GPS device lost signal for thirty seconds, I would rather show a gap or a clearly interpolated section than draw a confident straight line across data that was never recorded.

In VSDC, a practical approach is to import the external telemetry separately and use matching events to establish the video-to-data relationship instead of repeatedly sliding the track by eye. Once the offset is correct, keep the telemetry editable so you can still change gauge units, layout and route-map styling after the picture edit changes.

I would test one representative clip before processing the whole trip: trim it, split it, move it on the timeline and confirm that the overlay still refers to the intended source time. That catches synchronization assumptions early.

One more thing that saved me on a road-trip project: write down which clock each device uses. My camera, phone and watch were not all set to the same time zone, and one app exported local time while another treated the timestamps as UTC.

The data itself was fine. The confusing part was the interpretation of the timestamps. Once I stopped treating the file's displayed clock time as ground truth and aligned a visible event, everything became much simpler.

That gives me a better plan. I will keep the FIT as the archive master, export GPX only when needed, and sync each camera clip against one obvious event instead of trying to force the start times to match.

I also like the idea of testing trimming before doing the full ride. My previous workflow rendered gauges into an intermediate video too early, which meant every layout change required another render.