Hi Angela,
How exciting that you will be visiting Walt Disney World for the very first time! You are going to have SO much fun!
Disney Gift Cards can be used to purchase park tickets at Walt Disney World and I don't see any reason why you wouldn't be able to part pay for your
Mickey's Not-So-Scary Halloween Party tickets using your gift card and then pay the balance on a credit card.
That said, I would personally recommend that you purchase your tickets in advance from the Walt Disney World website. That way, you will be able to take advantage of the discounted ticket prices available when purchasing in advance and will not run the risk of tickets being sold out when you arrive at Walt Disney World.
Tickets can be purchased online
HERE and I would advise you to select the New! Disney eTicket option from the delivery options which means that your tickets are immediately available and can be printed on your computer.
Please let me know if I can be of any further help.
Wishing you a magical holiday filled with memories which will last a lifetime!