I am not sure you are hoping you COULD use part of a gift card and use the balance in a future purchase.
If that is your question or if it is anyone’s question, you can right now.
Add as many gift cards as you need to exceed the balance, the last card added will take only as much as is required to complete the purchase.
The balance of that purchase will remain on the card. Make a note of which card this was (you can check each card, but, that gets boring).
When you go to but something next time, start with the partial balance card.