I think Gatland is just being stupid saying this. It's not like the Lions will arrive in SA and immediately play the tests. There's going to be more than enough matches for him to test things out. So what if the Premiership players misses a warm-up match against Japan outside of a test window??
And even if he has a camp the week before in Wales, that shouldn't be held against the players. He can still have his camp while the players also still have time to be with their club preparing for the final. The Springboks have had these camps on many occasions, which usually starts on a Sunday and ends on Tuesday, giving the players more than enough time to still go back to their club and prepare.