Well, there is also the possiblity that Gale is not lying, but neither is he telling the truth because he doesn't know it, which is that:

his lover Mystra was actually Absolute masquarading as her. The Absolute bamboozled him to obtain the netherese orb, which was then used to modify the tadpoles. Gale wakes up on the ship with the other characters and assumes he was infected then, but he could have been infected much earlier, just not aware of it like the other true souls. Might be why he has better mental shields & control of his tadpole, as he has been carrying it around much longer.