GFO does not absorb alkalinity.
That said, there can be extensive precipitation of carbonates on the solid GFO, and elsewhere in the system. Such precipitation can, for example, be a contributing factor in the caking of such materials, and can coat other surfaces in the aquarium. This precipitation can also contribute to a drop in alkalinity and possibly pH as it removes carbonate from the water column. The effect of calcium will be similar, but smaller.
Just FYI.
This is why testing and dosing are necessary...and if you use GFO, you really can't just stop, and if you just started, you have to watch ALL your levels and increase dosing accordingly.