It is known that corrupted items cannot be altered with currency items. Does this apply also to quality?
Can I upgrade quality of a corrupted gem with a GCP? Can I upgrade quality of a weapon with a whetstone?
Yes, you cannot alter a Corrupted Gem with GCP (Gemcutter's Prism).
Because an item's quality is an alternation that boosts the damage/armor/etc. of an item, it cannot be used on Corrupted Gems.