Many times while playing against Gouken, I will attempt to standing tech his backthrow (important to note that I say STANDING tech), yet I will still eat the throw. I know that if you crouch tech in general, you take the risk of doing it too soon (before the throw connects), and you may instead get thrown out of your low short. But with standing tech, this shouldn’t be an issue because even if you get grabbed during your throw, the game just does a tech throw.
So anyway, I reproduced it several times in Training Mode, and my guess is that if Gouken’s backthrow connects during the startup of your throw, it will NOT result in a tech throw like it does with everyone else. The only way to tech his backthrow is if your grab connects first (which technically means he’s teching YOUR throw), or to wait until his grab actually connects and THEN tech it.
What’s good to know is that if you do get backgrabbed during the startup of your throw, the game still allows you to tech, you just have to push LP+LK again. And no, you still can’t tech if you get grabbed out of low short. So I think the best advice if you are going to tech Gouken’s throw is to mash the tech, and NEVER go for crouching tech!