Good job with the balancing.
The new ability is really nice and fits Gravity element of order very nicely. However I fear that the ability is powerful enough to rate a higher Casting Cost.
Quantum Pillars, Gravity Mark
Turn 5 activate ability convert 15 quanta to Gravity per Quantum Pillar and 3 more per pillar per turn (-1 per turn).
The quantum pillars are likely to generate
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to make this faster.
I think this is the best one from your Gem series and I would rate a balanced version of this on par with a balanced version of Soul Collector, Nova and Immolation.
So the ability is powerful but it fits as a thematically appropriate special quanta accelerator. I would keep it tied to the mark/Gravity (less randomness from a Gravity quanta generator) but increase the costs or maybe lose some of the quanta in the process?