How can I transfer money on my gift card balance to my Apple Mastercard account?

Normally once funds are in Apple Account balance the only way to get them out again is to spend them.

From: Check your Apple ID balance - Check your Apple Account balance - Apple Support -->

"About unused Apple ID balance

You can't redeem or return Apple Gift Cards, App Store & iTunes Gift Cards, or unused Apple ID balance for cash, except as required by law. If your jurisdiction allows for refund of redeemed gift balances, you can contact Apple Support." Read the support article for more details.

The above excerpt only mentions balances from gift cards as being eligible for refunding, and only in limited places where laws may require that. Read the support article for more details.