In general, revolver itself is somewhat deprecated for sustain right now on mana-using AP carries because Catalyst the Protector tends to give superior sustain and protection against burst in situations where you're having to deal with heavy harass; you typically only see revolver built on manaless mages for that reason.
It's not that gunblade is in itself a bad item, it's that you're spending a lot of gold on a relatively small gain; in general, an early Rod of Ages would accomplish much the same thing while making you significantly tankier in teamfights, and the short range of the active makes it far less useful to a ranged champion since you can only use it to disengage. Gunblade is built on mages with additional AD scaling, short ranges, and high burst - you tend to see it on Akali, Katarina, Mordekaiser, etc.