the GPS probably also wants power from USB
No the GPS is self powered (Garmin eTrex in this case).
The camera can receive commands from USB, so it should be
able to read the data stream, that a GPS device sends as well.
I indeed do geotagging with a seperate programe.
However it would be much easier if the camera did it itself.
It would remove the local time zone problem.
You have to set your camera to the local time, not UTC (GMT)
unless you want new year's firework shots from Sydney
to show up as shot on Dec 31 14:00 .