How can I use an Apple gift card for in App purchases on iPad?

Okay.

Redeem your Apple Gift Card - Apple Support

Use an Apple Gift Card in iTunes on PC - Apple Support

Redeem your App Store & iTunes Gift Card - Apple Support

Good Luck!

Before I post links to help you.

Can you, please, confirm what year and model iPad do you have/own?

The “A” model number can be found on the aluminium back of your iPad, in very small print, right under the larger bold print “iPad” on the back of the iPad.

What iOS/iPadOS version is this iPad, currently, running?

What is the rated, internal data storage space/capacity of your iPad model and how much of this free, internal data storage capacity is left inside of your iPad model?

All of that additional info can be found in the Settings app, General settings panel, About.

Please try to make an attempt to answer all questions asked.

Your gray colored hardware listing, at tge bottom of your post, suggests/indicates you have/own an obsolete, 10-1/2-year old, 2011 iPad 2nd generatipn.

There are almost NO MORE third party apps for such old iPads, any longer.

Thank You.