duden.de/gegenüber says the preposition «gegenüber» is followed by Dativ. I stood under the impression that it was followed by Genitiv in the past but found no mention of it in the respective article. Was it never used with Genitiv? Does someone know of a dictionary that suggested it's use with Genitiv?
«People using a genitive are simply wrong, possibly due to hypercorrection (the genitive is slowly being lost and replaced with the dative generally, so it happens that someone thinks a dative should really be a genitive even when it shouldn't).» - Kilian Foth