That is a bad bad idea. The bank is not supposed to accept a deposit for a person whose name is not on the account, so they will send it back to the IRS. Then the IRS will mail a check to the address on your return. All of that will delay your refund for weeks. And.....sad to say....if GF decided to keep your refund, you would get no help from the IRS because you told them to deposit it in that account.
Open your own account or use one of the other options for receiving your refund.
https://ttlc.intuit.com/questions/1901159-can-i-deposit-my-tax-refund-into-someone-else-s-account