Gunblade interactions?

Now I never use gunblade. but it can give spellvamp So I was wondering how it interacts with these skills... W and with gunblade, would that mean I would have a pre rework bulwark? where he shields himself and getting hit heals him a bit ult:this and rammus W/thornmail confuse me. as I wouldn't really call it AOE damage so I don't know if its spellvamp is reduced. if it isn't then a late game karthus ult would give him a ton of HP passive. now it doesn't show itself doing damage but if you play an ivern game and look at damage done you prolly did 100K+ true damage to jungle creatures anybody else know some questionable interactions?