So, after nearly two weeks of troubleshooting that included calibrating flow, retightening belts, trying out new nozzles, trying other spools of abs-gf, fucking around with extrusion multipliers, pressure advance values and temperatures, rebuilding the toolhead, changing out the hotend altogether, it turns out that abs-gf25 is susceptible to moisture, and it affects it quite a bit. I am disappointed to find what I thought would be one its main advantages isn't, and it was a stupid assumption in the process of troubleshooting.
I opened a new spool today and the difference is night and day. I'll try chucking the other spools in the dryer overnight.
Moral of the story I guess is just be careful of the assumptions that have been made.