For the first game of their South African tour, Lions head coach Gatland has made 14 changes to the starting side that beat Japan in their warm-up game in Edinburgh.
The only man to keep his place is Wales and Cardiff Blues wing Josh Adams.
Rees-Zammit, who had a fine Guinness Six Nations as Wales won the 2021 title, comes in for his first Lions start while his fellow countrymen Wyn Jones and Taulupe Faletau also begin the game.
Scrum-half Gareth Davies is also in line for his Lions debut from the bench.
Scotland full-back Stuart Hogg captains the side.
“It’s good to have arrived in Johannesburg and get the tour underway,” said Gatland. “We had a pleasing win last week up in Edinburgh, but there is still plenty we need to get right on Saturday.
“I’ve been encouraged by the way the boys have trained this week. I feel it’s gone up a level from where we were in Jersey and we’re starting to see a better understanding of some of our tactics.
“There are some new player combinations for us to have a look at this Saturday – which is good information for us as the Test series approaches.
“My congratulations to all those playing their first game as a Lion and also to Hoggy too – who I am sure will lead the side by example.”
Hogg, Adams and Rees-Zammit make up the back-three with England’s Owen Farrell and Scotland’s Chris Harris combining in the centre.
There is an all-Scottish halfback pairing of Ali Price and Finn Russell.
In the forwards, Wales loosehead prop Jones is joined by hooker Jamie George and Kyle Sinckler in the front row.
There is an all-English second row of Jonny Hill and Maro Itoje with No 8 Faletau joined in the back-row by Courtney Lawes and Hamish Watson.
Joining Davies on the bench are Luke Cowan-Dickie, Mako Vunipola, Zander Fagerson, Iain Henderson, Sam Simmonds, Bundee Aki and Elliot Daly.
THE LIONS v THE BRITISH & IRISH LIONS
Saturday 3 July 2021
Emirates Airline Park, Johannesburg
Kick-off: 5pm (BST)
15. Stuart Hogg – captain (Exeter Chiefs, Scotland) #783
14. Louis Rees-Zammit (Gloucester Rugby, Wales)
13. Chris Harris (Gloucester Rugby, Scotland)
12. Owen Farrell (Saracens, England) #780
11. Josh Adams (Cardiff Rugby, Wales) #836
10. Finn Russell (Racing 92, Scotland) #835
9. Ali Price (Glasgow Warriors, Scotland) #843
1. Wyn Jones (Scarlets, Wales) #842
2. Jamie George (Saracens, England) #819
3. Kyle Sinckler (Bristol Bears, England) #814
4. Maro Itoje (Saracens, England) #825
5. Jonny Hill (Exeter Chiefs, England)
6.Courtney Lawes (Northampton Saints, England) #826
7. Hamish Watson (Edinburgh Rugby, Scotland)
8. Taulupe Faletau (Bath Rugby, Wales) #779
Replacements:
16. Luke Cowan-Dickie (Exeter Chiefs, England)
17. Mako Vunipola (Saracens, England) – Prop #787
18. Zander Fagerson (Glasgow Warriors, Scotland)
19. Iain Henderson (Ulster Rugby, Ireland) #808
20. Sam Simmonds (Exeter Chiefs, England)
21.Gareth Davies (Scarlets, Wales)
22. Bundee Aki (Connacht Rugby, Ireland) #837
23. Elliot Daly (Saracens, England) #822
