Home loans

 Scenario: Property cost  $240,000  50/50 split ownership; Mom contributed  $100,000  owes $20,000 .  My mortgage is $140,000  - when she gathered balance  Mom will pay $20,000 towards mortgage so we each pay $120,000.  Is that $20,000 a gift or considered the balance to her half of the property? Deed is in both our names and check is made out to the bank.  If it is a gift who pays gift tax?