Big smile and be totally, over the top, super polite, this confuses your average gorilla, which makes them more angry. Then maybe a big smile when you pull away. You'll be laughing, he'll be having a coronary.
Your average large bodied, small brained, member of the Great Ape family wants a rise out of you, deny them the pleasure. It should be noted, apes have the smallest d*** of all animals which move around on their hind legs*
* this may not be true.