I've EQ's a Neighbor's pair of GC-2s in series more than once successfully with my 13 to 19 volt meanwell power supply. It did not take a huge amount of time, nor were the cells wildly out of sink with each other, they were just ~0.010 lower than I knew they could be.
I make extensive use of voltage bucker modules and voltage booster modules and some combination buck/boost models, mostly to speed/brightness control fans or LEDs, but also for batteries different than 12v. Most of these have current limiting potentiometers too.
One can easily follow 5 amps per 100Ah of capacity till 16v is achieved with such a bucker device, no homemade inline 12v lightbulb current limiting devices necessary.
If charging 6v gc-2s individually were my objective, I could easily accomplish it without some 6v dedicated charging source, using a bucker, and I can and do charge 24v batteries from 12v sources too, using a booster.
Heres a link to a 20 amp bucker with current control.