The problem here seems to be LR is making assumptions about time zones that aren't correct.
If camera time = GPS time there shouldn't be a need for any offset. It ought to be that simple.
It is that simple if the camera clock is set to UTC.
GPS data always uses UTC (GMT) time. The EXIF spec doesn't allow for time zone data so geotagging software doesn't know what time zone the camera clock is set to - it has to guess. Exiftool assumes it is the same as the system it is running on and compensates accordingly. I use exifttol for geotagging and if I am doing the geotagging in a different time zone to where the photos were taken I have to remember to change the system clock to match the photo time zone.