Hi,
Hopefully I'm not posting at the wrong place (please just remove it if mods feel it's inappropriate), but as someone who just geotagged hundreds of my trip SLR photos pretty accurately using this powerful tool, I'd like to share my experiences (with some minor tweaks to the `gpx.fmt` file) with other users - I documented in my blog: https://houtianze.github.io/photography/camera/slr/gps/geotagging/geo/2023/04/29/a-poor-man-s-dslr-gps.html
TLDR: For iPhone, you need to use `datetimeoriginal` instead of `gpsdatetime` in `gpx.fmt` and use the `-globaltimeshift` parameter to compensate the timezone.