But nothing explicitly states that you stop playing as the character you were playing as if they were transformed, whether into a playable character option or not. There is no rule saying that that character is playable, but there is no rule saying you can’t play as them. Again, spells do what they say they do.
No, you’re not understanding. There is nothing saying, RAW, you can’t play as a gnoll. The RAW is that you can’t choose a gnoll as your race - but in the case of being polymorphed into one, you didn’t choose that, you were changed into one. And the RAW for that is to assume that everything works the same as before, barring what is explicitly said in the spell for that transformation. And once again, nothing in the spell says you stop playing as a character who is transformed, regardless of if the transformation is “playable” or not. There is not even a “playable” tag for anything, anywhere, in RAW.