How can I get the ADS-B GNSS Antenna Offset and Position Offset?
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).