How to check my Apple Gift Card transactions

Since we have no way to access your account, I suggest that you contact Apple directly as they will be able to see any transactions.

Contact Apple for support and service - Apple Support

Explain to them that it is a Gift Card that has been added to your Wallet and you do not know what transaction caused the amount to be reduced.

Thanks for the update. The same will be true for a physical Gift Card after it has been redeemed to the Apple Account.

Here is the order that Apple uses to bill the charges:

1. Apple attempts to use your Apple Account balance to cover the full amount. What you can buy with your Apple Account balance.
2. If you don't have Apple Account balance or you don't have enough to cover the full amount, Apple attempts to charge your primary payment method. If you use Family Sharing, Apple charges your personal Apple Account balance before the family organizer's primary payment method.
3. If there's a problem with the primary payment method, or if the primary payment method can't be charged for the full remaining amount, Apple attempts to charge your other payment methods in order from top to bottom. Your primary payment method is at the top. When managing your payment methods, tap Edit to reorder a payment method. 
4. If none of them can be charged, Apple might try again.

How apps, content, and subscriptions from Apple are billed - Apple Support

What type of Gift Card do you have?

What type of gift card do I have? - Apple Support

If it is the App Store or iTunes Gift Card, the full amount is redeemed into your Apple Account. It is not possible to redeem a portion of the Gift Card. Are you viewing the amount that you see in your Apple Account?

If it is the Apple Store Gift Card, there is a number on the card you can call for assistance. These cards can only be used at an Apple Retail Store or online store.

Where are you viewing the Gift Card balance?