I'm reading the FAA's Advisory Circular AC No:20-165 appendix 1 point 28, which talks about GNSS Antenna Offset and Position Offset.
How would I receive this information from an aircraft? Is it transmitted?
The fields are transmitted when the aircraft has ADS-B protocol version 2 (assuming you are looking at 1090MHz) and is on the ground.
They are specifically added to the ADS-B MOPS (minimum operational performance standards) to allow for accurately depicting aircraft on the airport surface.
I don't know by heart which message type it is in, which bits are used and what the encoding is.
Best is to obtain a copy of the MOPS; RTCA DO260B or EUROCAE ED102A (they are essentially the same, get the cheapest).