How do I get refund on a pending purchase

If it's 'pending' you haven't paid for it yet.

Purchases are considered final but you could try the 'report a problem' page to contact iTunes Support and see if they will cancel it as a one-off : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via : https://support.apple.com/contact or https://www.apple.com/emea/support/itunes/contact.html

But you might have to pay for it before they will potentially refund you

If I was to purchase a iTunes or Apple gift card would the pending purchases be payed off

If you redeem an iTunes or App Store gift card onto the account it should be, you won't be able to use an Apple Store gift card