The plugin gets a gps position by searching the closest datetime the gpx file.
BUT generally the gpx datetime is different than your camera datetime. In this case you have to set the dtoffset. dtoffset is the difference (in seconds) between your gps tool date and your camera date.
You can check the date inside your gpx file and your images file and get the difference in seconds, or try 3600 or -3600 or 7200 or -7200 (+/- 1 hour, +/- 2 hours) and so on untill you get the images in in the correct place
This is a working sample: