Hextech gunblade not good on LB.
I tried to several times, and it's just not worth it unless you are really in late game.
First, it is too expensive. It costs like 3200+ and you are better off getting rabaodn(35% extra AP) or even Lich bane(10% CDR, 8% MS, 80 AP). I mean, you have no reason to get any AD stat.
Second, it doesn't really go with Leblanc's concept. Her job is to kill enemies and poof back to safety, but getting hextech gunblade means youa re oging to stay near the enemies for prolonged time...but unlike Akali or Kat, LB can't do that because hse doesn't have any skill cooldown reset mechanism.