Does Garmin G300 GPS use true heading or magnetic? Sorry couldn't find this information anywhere else.
This document from Garmin states on page 265 that headings are magnetic headings. Grid lines shown on the terrain display are oriented to True north, but that is the only reference in the manual to true north. All other occurrences of "true" in the manual refer to True Airspeed.
Which is to be expected; nearly all headings in aviation are magnetic headings. Courses can be true or magnetic, but they're converted to magnetic before they're converted to headings. (Yes, grid navigation near the poles uses other than magnetic headings, but those aren't oriented to true north either, and this Garmin doesn't do grid navigation.)