It's very normal here, don't worry. ;) I have a few that will know him!
First is obviously Rhys! He is very excited to have both of his siblings around. Gareth was obviously a bit older than Brynn when Rhys became their legal guardian, so I'm assuming that their sibling relationship might have been a bit different. He was still protective of his brother, but given that he was already well into his teens, he probably didn't have to be as hands on with him, I'm guessing? He tried to keep from being too fatherly to them both since he knew he was still their only brother despite what he might have been legally, but y'know. He's also pretty open about his dreams and has talked about them vaguely in public and more extensively with Brynn so Gareth could know about them as well, if you want!
Liam (~wrmtail) will also know him! He likes him, because he makes Thea happy and that's really all that matters to him. He is not and never will be the overbearing, "if you hurt her, I'll hurt you" type and wouldn't even pretend to be, so he was basically welcomed into the fold with open arms.
As mentioned in chat, the Caroline Wolfe works as a hairdresser at Miss Belle's, so the Wolfe kids are in and out of there pretty frequently. She babysits Theo's daughter, Zoe, when he's running errands and she's usually hanging around on her iPad or getting her nails painted. But yeah, Theo (~winterfalls) is a friendly guy. He's also besties with Max, who has all sorts of ties to Rhys and Lyllia, so he probably would have met Gareth in one way or another outside of the salon/spa, too.
Brynn is one of Eve's (~maxcooking) best friends! They went to the same school in NYC and have been friends since childhood, so she'd probably know Gareth pretty well by now.