A dedicated Mac geotagging app is the simplest option. Based on the suggestions here, two good choices are GeoTag and HoudahGeo.

Both are designed to match your photos’ timestamps with the positions recorded in a GPX track and then write GPS coordinates into the images’ EXIF metadata. GeoTag uses ExifTool under the hood for the metadata writing. HoudahGeo is especially well suited if you need bulk geotagging and tools to correct time offsets, time zones, or manually refine locations on a map.

In general, the workflow is:

1. Import your photos.
2. Load the GPX track.
3. Make sure the photo capture times are correct.
4. Apply any needed time/time-zone offset.
5. Let the app match photos to track points.
6. Write the GPS data back to the files.

If your camera clock was off when the photos were taken, software with time adjustment features will make the process much easier.