My experience with gyro instruments is they fail in the first few weeks or go on for a long time. I have had a number of instruments overhauled by the company linked above, one did fail quite quickly and they fixed it almost by return of post with no quibbling.
As to the GPS source for you Garmin you might be able to use the output from the GPS unit that Trig fit to their transponder ?
The other option might be to fit one of the FLARM / ADS-B boxes and use the GPS output from that , the advantage would be traffic information, especially useful in detecting gilders.
if you can find Wigglyamp on these pages he is the bloke to ask.