I don't even eat gluten.
B is joking.
B makes the assumption that A suspects him to have something in the B's sleeves, like assassins could have a knife or a poison.
Then he denies his own assumption, saying "That's not my style". Because from the context B must have poisoned either both or none of the drinks, but obviously B wasn't going to die.
Moreover, B cares of his health to the point that B "doesn't even eat gluten".