Today I got to the project, armed with RB5 relay board.
Opened the enclosure and searched for a siren output or the like without any luck.
Looked at the board once again and noticed that it wasn't the same as what Ryan linked to. This is the board that they have: [Link: apollogate.com]

What I ended up doing is getting power from the gate motor (black=ground and black= +12V when gate opens/ -12V when gate closes).
The relay powers on the gate open cycle, opens N.C. relay output which in turn is tied into the HAI.