My understanding was that the internal antenna wasn't allowed to be used for certified operation, as per the STC - it needs connected to a dedicated antenna to drive its internal receiver or get GPS data from another unit. Non-certified it can quite happily use its internal antenna.
Garmin G5 Electronic Flight Instrument Part 23 AML STC Installation Manual 190-01112-10 Rev. 11:
3.4.1 GPS Antenna
A GPS signal is required. The G5 can use an external GPS antenna or GPS data from one of the GPS navigators or GTX 3x5 listed in Section 5. In a dual G5 installation only the G5 Attitude requires the GPS position input. The GPS information is communicated to the G5 DG/HSI over the CAN Bus.
Worth saying that certified & non-certified are identical units - you say whether its certified or not on a menu during system setup.