.. How do I find Gavin?

You don't. He finds you.

You'll hear the name first from Madison, Penelope's daughter-in-law --

Penelope Haines

-- and every other person you mention it to is either clueless or traumatized. This is clearly a very bad man (or whatever he is).

After you finish investigating Tiffany and Jamie, give the report to Lisa. Gavin will meet you when you leave her place.

Lisa Tenzin