Roberts missed Saturday's 23-21 victory over the Wallabies at Suncorp Stadium because of a hamstring injury, but the Wales centre could figure in selection when the series resumes in Melbourne.
The news continues a positive series of medical bulletins that has seen Manu Tuilagi and Tommy Bowe regain fitness for the second Test. Tuilagi, who has been sidelined with a shoulder problem, starts Tuesday's match against the Melbourne Rebels at outside centre while Tommy Bowe has been given a clean bill of health following hand surgery.
Gatland paid tribute to the Lions' medical team with the tourists close to declaring a clean bill of health. Remarkably, only Roberts and prop Alex Corbisiero, who has a tight calf, have question marks hanging over their fitness with the rehabilitation of Bowe and wing George North completed well ahead of schedule.
"The first prognosis of George North was that he would definitely be out of the first Test, so the medical team have done a fantastic job getting those guys back fit," Gatland said.
"To have Tommy and Manu fit and back is a great compliment to the medical staff. Tommy's fit for Saturday, he's running, but the more time he has to heal the better. The surgeon said that that injury is normally a six-to-eight-week injury, so to be fit in three weeks is magnificent. It's important not to risk him on Tuesday."
England lock Geoff Parling will lead the Lions against the Melbourne Rebels at AAMI Park on Tuesday night. Rory Best drops to the bench following his poor performance in last week's 14-12 defeat by the Brumbies in Canberra. The whole bench from the victory against Australia is involved against the Rebels, although none of the starting XV feature.
British and Irish Lions team to face Melbourne Rebels at AAMI Park, Melbourne, on Tuesday, June 25 (KO 1040BST):
R Kearney (Ireland); S Maitland (Scotland), M Tuilagi (England), B Barritt (England), S Zebo (Ireland); O Farrell (England), C Murray (Ireland); R Grant (Scotland), R Hibbard (Wales), D Cole (England), R Gray (Scotland), G Parling (England, capt), D Lydiate (Wales), S O'Brien (Ireland) T Faletau (Wales).