I am well aware that Gabriel does not really know who either of his foes are, and certainly acts like it, but there was a scene in this episode that has got me wondering about it.
First though, I'll bring up the episode Simon Says. Now that we know for sure that Gabriel is Hawk Moth, we know why he was showing such strange attention to Ladybug's earrings and Cat Noir's ring. But in the end, when he was hugging Adrien, we see him stiffen and gasp as he sees Adrien's ring. He says in almost a stunned voice that he's never noticed his ring before. Adrien doesn't give him much of an answer, but we know why Gabriel was so shocked: because he knows Cat Noir wears his ring on the same finger of the same hand. That's probably where his supicion started. I almost expected him to go a little less hard on Cat Noir in the following episodes, but there was none.
However, towards the end of Collector, Gabriel shows a burst of sudden worry for his son after Cat Noir tells him what had happened to him (although there's a possibility he was faking it). When Cat Noir hastily told him that "heeeeeee.... must be hiding," (XD) Gabriel walks up to them and tells them that Adrien is too precious to be lost to him, but oddly, when he approaches them, he comes straight to Cat Noir and puts his hands on his shoulders, almost as if he was meaning something else... Cat Noir seemed happy with it, like he was glad to know just how much his father cared for him, and is simply smiling up at him while Ladybug freaks out over her declining power. Anyway, I just found that scene very strange, and so I think that maybe, maybe, he actually knows.