Hextech Gunblade Heal

So I'm aware that heals for "15% of all Damage Dealt" but it also seems that this isn't true of something like 's E for example.

Does the heal work with onhit damage such as, , , , , and really anything else that cause some kind of additional onhit damage whether it be Magic or Physical.

Been trying out in the jungle, and obviously his W, in addition to rageblade, devourer, causes a massive amount of damage in a fight, but the heal doesn't seem to hold up. Trying to figure out what item is the best for his sustain, because isn't cutting it, even with a gunblade.