Oh, there are definitely hints about it in earlier conversations, but:

Gale's love of magic and his love for Mystra are deliberately muddled in said conversations. It's understandable, the two things are probably very muddled for him, too, especially since she is magic and all. But the result is that it's not at all obvious he's in love with her as opposed to a general wizardly obsession with sparkly magic stuff (note that if you play a wizard, the PC doesn't notice anything odd or different about the sensations during the Weave scene, it's all exactly as they've experienced it before).

Also, the number of people in the Realms who have actually romanced a god is... small, to say the least. As a result, the leap from "this wizard totes adorbs magic" to "this wizard was banging Mystra" is a pretty big one; few people are going to make it.

He does say that he's not sure whether he still loves her, but he's obviously still hung up about it and conjuring her image and stuff. And saying, "I won't know until I see her again" is not very helpful. He might never see her again - where does that leave his relationship with the PC? And if she happens to pop up again tomorrow, is he gonna go, "Oh, I guess I'm still in love with her, see ya!" and hang the PC out to dry?

No thanks, Gale.