I have an RV 7 and thats where eI put mine just where Terry said.
The other acceptable option is in the wing tip.
I believe in the Garmin G3X Install manual it lists best locations for the GMU 11.
Appendix B form the manual;
B.2 Van’sAircraft
B.2.1 RV-7/9
The typical GMU magnetometer mounting location in RV-7/9 aircraft is in the aft fuselage, on the aft deck (F-714), just ahead of the horizontal stabilizer and just aft of the F-709 bulkhead. This location provides a level pitch and roll reference, and acceptable separation from excessive magnetic disturbances in typical RV-7/9 aircraft.
The wingtip can be used as an alternate GMU magnetometer mounting location for RV aircraft. Ensure adequate separation from steel aileron counterweights, transmitting antennas, and lighting fixtures, and follow the wingtip light wiring guidance for in Section 15.4 or Section 16.4.