Nashors isn't going to give her much extra damage for how little AP you'll have. Plus you'll have no magic pen. If you want AS or more on hit damage get a bork. If you want CDR, get lucidity boots, switch out the BT for an ER, and trade the nashor's for a bork. You don't have enough AP or any magic pen to be a mage fortune with that and you are hurting your AD for sub par items.
Gunblade isn't that great on adcs in general. Spell vamp sucks and that stats aren't that great overall. The only champ who can really justify a gunblade atm is akali, but some others like kayle and poppy have potential with it. If you want AP and AD on the same item, get TF instead of a gunblade.