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.