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

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

Once you begin to cast the Zombie spell, losing control of Gisa and Geralf won’t affect the spell.

If you cast one Zombie creature spell from your graveyard and then have a new Gisa and Geralf come under your control in the same turn, you may cast another Zombie creature spell from your graveyard that turn.

If a Zombie 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.

If multiple effects allow you to cast a Zombie creature spell from your graveyard, such as those of Gisa and Geralf and Karador, Ghost Chieftain, you must announce which permission you’re using as you begin to cast the spell.