I think he’d do both.
He’d seduce Credence slowly, being so careful of his boundaries and never to push him, never asking for anything he wasn’t comfortable with. He’d be patient and so very kind, giving Credence soft touches and working up to kisses, never ever doing more than he could handle.
And he’d watch Credence play with himself at night, so achingly aroused by seeing his tentative, exploratory touches as he learns his own body. Credence had always been taught that touching himself was a sin so he’s rather inexperienced, but Gellert finds his naivete in that area incredibly attractive. It feels so intimate to watch him discover pleasure.
He finds himself looking in on the boy quite often, and before long he notices a pattern – Credence is much more likely to return to his rooms and start touching himself after Gellert gives him a nice, deep kiss. On the day Credence cries out his name when he orgasms, Gellert comes harder than he has in years.
And when Credence finally finds himself in Gellert’s bed, he knows just how Credence likes to be touched? All the right places, exactly the right pace, how??