GMT offset is provided so that if you travel outside your time zones you, can correct the displayed trip time.
Satellites send GPS data that is at GMT = 0:00
i.e. Greenwich Mean Time. This time stamp is saved with every log.
When the logger's data is synced into a database, the computer calculated the offset in time from your time zone.
In other words, your computer time must be correct , so that the real time is calculated.
This is only used in very specific reasons.
If you live in New-York, and you used your logger while in Los-Angeles on a trip.
Say you were driving at 10am
When you retuirn to New-York, and you sync to updated your database in the East Coast.
The trip time stamp for those trips will be 3 hours off.
To correct one or many trips, simply highlight them using Shift-Click and then right click to select GMT offset
and set the correction in the time.
Trips will then be converted to the real local time it was done. and not the time where you sync your logger.
If you had driven to a customer in LA at 10AM local time, without the GMT correction, the time of the trip will be 1PM when you download it into your computer, as this will be the time zone of New York. The same time zone in your computer , since you live in New York.