The 31-year-old centre-half has signed a one-year deal with Rangers after his contract at Upton Park expired this summer.
Gabbidon had been training with QPR already and played in three friendlies on the club's recent tour of Cornwall before today joining Jay Bothroyd and Keiron Dyer in signing for Neil Warnock's side.
Gabbidon told the club's website: "I'm delighted to be here. "It's a good club. Hopefully I can learn more about it in the coming weeks. I had a good week with the lads in Cornwall and I'm pleased to have signed.
"My agent spoke to the manager and he said to come down and train for a week. It went from there.
"He (Warnock) was a big part in me joining the club. I've been quite impressed by him. He's straight talking, which I like. If you're doing something right I think he'll be the first to tell you and, if you're not, he'll be the first to tell you again.
"As a player, that's what you want. I was impressed with what he had to say about the club and what he wants to do this season.
"I think we can do well. The lads showed last season that they're already confident and hungry to do well in the Premier League."
The Welshman added: "Hopefully I can bring experience to the table having played in the Premier League for a number of years.
"I know the ins and outs of the league, and what's required. There are a lot of good players here already but hopefully, with my ability, I can add something to the team."
Warnock is delighted to add top-flight experience to the spine of his team.
"He's impressed me in the last 10 days or so that he's been training with us," he said.
"I think he's just the type of player we need - a quality defender with plenty of experience.
"I hope I can reinvigorate his career, because a couple of years ago I thought he was one of the best defenders in the country."