Gohan hadn't been able to move a muscle by the time the blast was repelled, so unless Gohan's reaction speed is somehow much slower than Tenshinhan's it has to be the stun damage.
Well, that's why I think the scene is just AT being inconsistent, just like Boo trying to prove a point on Goku being useless using Tenshinhan falling with a kick when he's fucking Super Boo with SSJ3 Gotenks on top lol. I think Toriyama just didn't think well enough those scenes.
Like, we had Gohan doing this:
All which shows incredible resilience and reflexes. He seemed affected to a point he had to pause upon his face being punched and scream in pain... Upon noticing the blast was about to hit, he still had the quick reaction to dodge it. He broke out the galactic donuts which tool an incredible amount of effort all while Boo was already launching the KMHMH on him... That shows incredible reflexes and resilience. I don't think the Kiai affected him that much considering he didn't seem really injured after it and hadn't marks of damage and pain like in the above situations.