Gem leveling is like, +10 or higher, 100%. Then every level under 10, you lose 10% chance, until you reach 60%. At which point, it stays there until you reach gem level -1, which drops to like, 30%, and -2 to like 15% or something like that and keeps being halved, until it reaches 0%(it used to cap at 1% but I think they changed that at some point, regardless it's shit at some point).
So as long as you do +10 you're fine, which means for lvl 25 gems, GR35 is enough(technically, GR34 is enough too, since you need 100% to go from 24 to 25, not at 25). That's Torment 6 or 7 I forgot, which should be doable with a full set, the right skills, and a legendary weapon, with nothing much else.