Sexton had undergone a scan on a hamstring problem which had not revealed a tear, Gatland added, and there was good news for Tommy Bowe, who underwent surgery on a broken bone in his hand which he suffered against the Queensland Reds.
Despite indicating that the Ireland winger would play no further part in the tour on Saturday, Gatland said it was now possible he might be available in two weeks and no replacement would be made immediately.
"The surgeon has spoken to him and said it could just be a couple of weeks actually," Gatland said.
"Once the plate's put in, the bone's been fixed basically."
Ireland's Simon Zebo would fly in from Houston, Texas, to join the British & Irish Lions as cover for Bowe, the team said in a statement later on Sunday.
Zebo would meet up with the squad in Newcastle ahead of the game against Combined Country, while Bowe's condition would be assessed over the next few days, it added.
The Combined New South Wales and Queensland Country invitational team, which will play the Lions in Newcastle, is probably the weakest side the tourists will play in Australia.
Gatland said he was starting to think about his test team and would probably play a majority of it in the match against New South Wales Waratahs in Sydney next Saturday.
For Tuesday's match though, Gatland has again rung the changes in the centres, reuniting Ireland's Brian O'Driscoll with Welshman Jamie Roberts in a partnership that served the Lions well in 2009.
"Really looking forward to it, obviously Jamie and myself got on well in South Africa four years ago," said O'Driscoll, who will captain the side as he did in the second match of the tour when the Lions pummelled Western Force.
"We clicked early on in training and we managed to do so in games as well. It would be nice to pick up where we left off."
Hogg played at fullback in Saturday's 22-12 victory over the Reds and will have no specialist back-up on the bench in Newcastle, although Gatland indicated fullback Leigh Halfpenny or centre Jonathan Davies would step up if the Scot was injured.
"We've said that he was covering that position," Gatland said of Hogg.
"Given that both 10s have been involved in every game so far, we're probably at the stage where we need to look after them a bit."
Scottish lock Richie Gray as well as Welsh wingers Alex Cuthbert and George North will also start just three days after playing significant roles in the Reds match.
Alex Corbisiero and Ryan Grant, the props called up in the last 48 hours in place of the injured Cian Healy and Gethin Jenkins, will get a first run-out at the Hunter Stadium.
England's Corbisiero starts in the front row alongside Richard Hibbard and Dan Cole, while Grant was named on the bench after travelling to Brisbane from South Africa.
"They've just got to get up to speed with calls and so on but it's nice to give them a start," Gatland said.
"You feel sorry for the players who are injured and have to go home but when you see the excitement in the players that come in, it's a lift not just for the players but for the coaches.
"(Alex) was like a small kid in a lolly shop."
British and Irish Lions: 15 Sean Maitland, 14 Alex Cuthbert, 13 Brian O'Driscoll (captain), 12 Jamie Roberts, 11 George North, 10 Stuart Hogg, 9 Conor Murray; 8 Jamie Heaslip, 7 Justin Tipuric, 6 Sean O'Brien, 5 Ian Evans, 4 Richie Gray, 3 Dan Cole, 2 Richard Hibbard, 1 Alex Corbisiero.
Substitutes: 16 Rory Best, 17 Ryan Grant, 18 Matt Stevens, 19 Alun Wyn Jones, 20 Toby Faletau, 21 Mike Phillips, 22 Jonathan Davies, 23 Leigh Halfpenny