I must have missed something....
anyway my point was going to be that it's said (now I know not everyone is going to agree with this, and I am not saying its a hard fact) that when babies are started on puree they learn to swallow in a new way (as opposed to how they swallow milk, with their tongue forward) to learn how to swallow pureed foods that are thicker than milk.
So when you start a baby who's had exclusively puree onto solids, they will automatically move food to the back of their mouth like they would with puree so they need to figure out how to manage this new food, that's all
Apparently some babies will gag more due to have to learn this, but not all. But gagging is learning, nothing bad.
Now, there is absolutely nothing wrong with this, as the girls have said. He's just working out what he needs to do with a piece of toast as opposed to a puree. So him gagging is him working it out. 99% of the time they will do it on their own, and it's all a learning experience