You must follow the normal timing permissions and restrictions of the spell you cast from your graveyard.

You must pay the costs to cast that spell. If it has an alternative cost, you may cast it for that cost instead.

Once you begin to cast the spell, losing control of Arcade Gannon won’t affect the spell.

If you cast an artifact or Human spell from your graveyard using another permission, Arcade Gannon’s effect doesn’t apply. You can cast another artifact or Human spell from your graveyard.

If you cast one artifact or Human spell from your graveyard and then have a new Arcade Gannon come under your control in the same turn, you may cast another artifact or Human spell from your graveyard that turn as long as that spell has mana value less than or equal to the number of quest counters on the new Arcade Gannon.

If an artifact or Human card is put into your graveyard during your main phase and the stack is empty, you have a chance to cast it before any player may attempt to remove that card from your graveyard.