The 32-year-old started his career at Widnes, before spending two years at the Hornets, and the last three in North Wales.
“He’s signed a contract to November 30,” said Oldham chairman Chris Hamilton.
“A few clubs were after him and we worked hard to get his signature.
“He’s a top player and we’re confident he will make a big contribution this season.”
Oldham coach Scott Naylor is pleased to be adding a player of such pedigree to his squad.
“Whenever we’ve played Crusaders in the past three years, Gary has been one of their top men,” he said.
“He works hard, plays the game tough – that’s how I like it – and he never takes a backward step. That is very important in the Championship.
“We are fortunate to get him. He will be a great addition to the squad. I’m looking forward to seeing him in the Oldham jersey.”
The player himself is looking forward to ripping in as soon as possible.
“I’m naturally fit, so it won’t take me long,” he said.
“I’m looking forward to a new challenge, a new club and a new set of lads.
“I used to watch Scott (Naylor) when he was a player, and now he’s going to be coaching me. I’m looking forward to that too.
“At 32, some people might say I’m getting on a bit, but I’ve got a couple of good years left in me yet.”