You can activate the last ability of Grand Master of Flowers even if there isn't a card named Monk of the Open Hand in your library or graveyard.

If Grand Master of Flowers somehow gains enough loyalty to become a creature while it is being attacked, it is removed from combat. If blockers haven't been declared yet, it may then block. It may even block creatures that were previously attacking it. Creatures that were previously attacking it that go unblocked won't deal combat damage.

The first ability of Grand Master of Flowers doesn't use the stack and can't be responded to. Similarly, players can't respond to paying the cost of adding loyalty counters for an ability. Therefore, if Grand Master of Flowers has six loyalty and you activate one of his abilities, he will be a 7/7 Dragon God with flying and indestructible before players can respond to the loyalty ability.

While Grand Master of Flowers is a creature, he is not a planeswalker. He cannot be attacked and damage dealt to him while he is a creature will not reduce the number of loyalty counters he has. However, you may still activate his loyalty abilities as normal.