1. Will my GCash details be saved as payment method for future purchase?
Your GCash details won't be saved which means you'll have to enter your it each time you would be paying for your Facebook Ads with GCash.
2. Can GCash funded balance be withdrawn if unused?
Any amount paid to Facebook stays on your Facebook account and can no longer be transferred back to GCash. To know you may visit Facebook Business Help Center for further assistance on your Facebook for Business account
3. Can I still refund payments made for Facebook Ads?
Facebook may allow refunds depending on their refund policy. To request for refunds or dispute any payments made, please visit their Facebook Business Help Center for further assistance