Gary, 48, shared the photo - taken in 1992 - to celebrate Robbie's record-equalling 13th career number one album, the most by a solo artist.
Wearing muscle vests and looking fresh-faced in the throwback posted onto Twitter, Gary captioned the image: "We pay tribute to the guy who’s number one for the 13th time @robbiewilliams.
"Here’s a shot from 1992 at a gig in Portsmouth. Congratulations mate. #lookatus."
And it wasn't long before many of Gary's 4.2million followers took to the social media site to express their joy at the snap.
One follower said: "You're so beautiful as always Gary sunshine, you're the sweetest.
"I love this picture."
Amazing. Love you two so much.
"Amazing. Love you two so much," another wrote.
Finally a third joked: "What do you mean? Surely that pic was taken last week."