Can "G" be used to further a regular chain or just reactive chains (Stinging Backfist)?
Its not just 'G'. 'G' will activate your most recently achieved Finishing ability for you when you crit with an ability (you have to press the hotkey for anything other than the one you most recently learned... its annoying, there should be a selector, but I digress.
'Y' will activate a defensive ability when you dodge (similar in function to 'G', its now the one you last learned)
'V' supposedly (nothing to test with) activates a rescue when it becomes available.