Kyle Sinckler, who on Tuesday had his citing dismissed following a disciplinary hearing, is also among the replacements.
The Lions won the series opener but were overpowered last weekend, with South Africa scoring 21 unanswered points in the second half to win 27-9. Coach Gatland has since overhauled his side.
Among the glaring areas of weakness for the Lions was the back three’s work under the high ball, and Wales full-back Liam Williams and wing Josh Adams have been brought in to fix that issue. They replace Stuart Hogg and Anthony Watson, who are both dropped from the 23, with Duhan van der Merwe keeping his spot on the left wing.
Replacing Scotland’s Chris Harris, Bundee Aki pairs up with his Ireland team-mate Robbie Henshaw in the centre, while Ali Price returns to replace the benched Conor Murray at scrum-half.
Up front, there are just two changes, both in the front row, with Welshmen Ken Owens and Wyn Jones, who is fit again, coming in for the benched Luke Cowan-Dickie and Mako Vunipola. There is still no place for Jamie George, a star of the 2017 series in New Zealand.
Adam Beard and Sam Simmonds come onto the Lions’ 5-3 bench for the first time this series, meaning there is no space for Tadhg Beirne or Taulupe Faletau.
The mercurial Scottish fly-half Russell is back from injury to replace Owen Farrell on the bench.
“The matchday 23 have an incredibly special opportunity in front of them – to seal a Lions Series victory in South Africa,” said Gatland.
“It doesn’t get much bigger than this and we’re excited by the challenge that faces us on Saturday.
“We’ve no excuses from last week, the Springboks put us under pressure in the second half and deserved the win. We have to be much better than that second half performance, and I believe we will be.”
The Springboks were also due to name their side this afternoon, and were sweating on the fitness of Pieter-Steph du Toit and Ox Nche.
South Africa escaped citings from an ill-tempered Second Test, but do have a disciplinary hearing hanging over them. Director of Rugby Rassie Erasmus will face a panel for criticising the performance of the officials in the First Test – but not until after the series concludes. He is therefore free to act as drinks-carrier for the Springboks this weekend.