Q: Where does Garmin recommend the GAP 26 pitot tube be located on the wing? Specifically, how many inches forward of the wing leading edge?
PER THE GARMIN G3X INSTALLATION MANUAL - The tip of the GAP 26 should be located at least 4" from, but not more than 10" below the bottom surface of the wing. The probe tip may protrude up to 2" max in front of the leading edge of the wing.
Q: Does this Garmin G3X GAP 26 unheated ptiot come with a mounting plate?
No, this does not. Please look at one of the options, the PBK-12.