32-year-old Reid - who made his Warriors debut away to the Ospreys in March 2011 - racked up 113 appearances during his first stint with the club, playing a key role in Glasgow's march to the 2015 Guinness PRO12 title in Belfast.
The Ayrshire native joined the Bulls ahead of the inaugural FOSROC Super6 season, following a spell with London Irish.
Reid will remain an Ayrshire Bulls player, but will train with Dave Rennie's squad and provide valuable cover at loosehead prop for the remainder of the Guinness PRO14 and Heineken Champions Cup campaigns.
"I'm excited to be back at Scotstoun," said Reid.
"The club means a great deal to me, so I'm happy to have signed a partnership contract until the end of the season.
"There are a few familiar faces and I'm looking forward to getting to know the new boys too.
"I'll still be playing for The Ayrshire Bulls, but will also be available for the Warriors, so it's a win-win situation."
Glasgow Warriors assistant coach John Dalziel said: "We're happy to have Gordy back involved with us at Scotstoun.
"He's a quality international who's just come off the back of a World Cup, and it's great to have him back in training with us alongside his contract with Ayrshire Bulls."