Simmonds, the Exeter number eight and current European player of the year, has been picked by Gatland despite being consistently overlooked by England since winning the last of his seven caps in 2018.
He is joined by fellow bolters and centres Bundee Aki of Ireland and Scotland’s Chris Harris, with Manu Tuilagi left out as he approaches his comeback from an Achilles injury.
However, Sexton is unable to force his way into the 37-man squad announced by Gatland, as are Jonathan Davies, Sam Underhill, Billy Vunipola and Josh Navidi.
Gatland said: “To those players that have been selected, congratulations. I know it’s going to be challenging for a number of aspects. I just hope firstly that they thoroughly enjoy the tour and represent their four home nations the best they possibly can.”
On naming 37 players rather than the expected 36, Gatland said: “Particularly looking at our loose forwards, and the attrition, getting an extra number there is a bit of a back-up. We wanted to keep the numbers down but a 37-man squad was something we looked at yesterday.”
Gatland knows taking on the world champions will be a huge challenge.
“It’s not going to be easy,” he said.
“It’s probably using the experience from the last tour. Back in 2009 we played pretty well in the lead-up but the physicality they brought to the first Test was something we weren’t quite ready for.
“We came away thinking we’d done a pretty good job, restored some respect back into the jersey. But we were disappointed, we felt we could have drawn or even won.
“We’re going back to South Africa with the ultimate goal of continuing what we’ve built on and building the Lions brand. It’s unique, it’s incredibly special.”