Mine does the same, so I’ll be interested to hear if you find a solution.
It may seem obvious, but wouldn’t this be simply the difference between a gyro behaviour and an electronic AHRS as used by the G5? Theoretically both should be the same, but given the technology generation differences …
The question about which to believe is certainly the most important. If the answer is the G5, then that begs the question why a TC is still required for a G5 equipped aircraft. Perhaps regulation not keeping up with technology?
I don’t know about the G5, but I have the Sandel SG102 AHRS and that needs to be mounted accurately along the aircraft axis.
If the mechanical ball is centred on the ground, with the aircraft levelled (there is a procedure in the MM for that) then I would trust the mechanical instrument.
I should add that I did think twice whether a dual G5 installation replacing King HSI and vacuum AH would be worthwhile in what is effectively an airborne farm bike, but after a first 290 nm flight yesterday I can heartily recommend it.
@boscomantico
Have you checked/adjusted the G5 roll offset as detailed in 6.3.7.1 of the Garmin G5 Electronic Flight Instrument Part 23 AML STC Installation Manual 190-01112-10?
Did that affect the yaw indicator as well as roll attitude?
Mine looks as if it needs a wee tweak in pitch and roll.
This has just been put up for sale on a US FB site
There is considerable debate around this practice, and I am not talking about the seller 
