I wonder if Gale's origin isn't actually connected to the main story in a major way:
Gale carries a netherese orb in his chest and it was netherese magic which was used to change the tadpoles.

So here is another guess (though I'd consider it very unlikely): his story is a fluke and Gale is
the other Shar worshipper the journal (currently due to a bug) mentions. Perhaps the orb was the source of magic that was used on the tadpoles.