It's all just basic Math, with the Striker class once you get out of level 3 your going to be doing 100 damage a shot. With the percentage ruin your going to do 4 extra damage. When you get to level 4 (Don't totally quote me, all the strikers do slightly different damage...) you do 125 damage and your next gem becomes active increasing your damage by 5. So it would still almost be worth it to use the +6 damage increase anyways. Once you get towards end game levels (10,11,12, ect.) Start using the % based gems. That's at least how I go about putting gems into builds.
By the way... FUCK