The 33-year-old last appeared for the national team in the spring of 2013 but will make his first Cardiff Blues start since overcoming his battle with testicular cancer on Sunday.
And Gatland, who led the Lions to a historic 2-1 series victory in Australia last summer, believes Rees could well be on the plane to South Africa this summer.
"Everybody was willing Matthew on in his recovery from illness and was delighted to see him return to the playing field," Gatland told Wales Online.
"He's in the best nick I've seen him in for a long time physically and the experience of Matthew and Gethin [Jenkins] has been important in the recent turnaround of the Blues.
"You only have to see the difference those two make with their knowledge, know-how, talking and cajoling a young team, and their sheer desire to win.
"Matthew is certainly one the Wales selectors will be watching on Judgement Day, when he's up against Ken [Owens] in a derby which we are viewing as an unofficial trial before we name the squad for the tour of South Africa.
"What Matthew has got is experience you can't buy and he's certainly in the shake-up to make the squad for the demanding assignments we face in the summer."