The most efficient way to get a level 25 Legendary Gem would be farming "9" Greater Rift Trial Keys, which drop off normal rift guardians/bosses. Than proceeded to complete Greater Rifts Level: 13, 16, 19, 22, 25, 28, 31, 34, 37; Which allows a 100% chance to upgrade your gem to tier 25 | PS. don't forget to hit upgrade gem every time you complete a greater rift