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 will have surgery on a broken bone in his hand which he suffered against the Queensland Reds.
Despite indicating that the 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 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 has continued to ring the changes in the centres, reuniting Brian O'Driscoll with Jamie Roberts in a partnership that served the Lions well in South Africa in 2009.
O'Driscoll will again captain the side, as he did in the second match of the tour when the Lions pummelled Western Force.
Hogg played at fullback in Saturday's 22-12 victory over the Reds and will have no specialist back-up on the bench, although Gatland indicated that Wales fullback Leigh Halfpenny, or centre Jonathan Davies, would step up if the Scot was injured.
Scottish lock Richie Gray and 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.
Team: 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.
Replacements: 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
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- Tommy Bowe
- Queensland Reds