If you co-signed for your GF's student loan, then you can use the interest from the 1098E on your tax return. Otherwise, you cannot use it on your tax return. Only the person who is legally obligated to pay the student loan (including a co-signer) can use the interest. If you are "gifting" her the money to pay the loan, a gift given to her is not deductible for you. If you give her more than $14,000 you need to complete a 709 gift tax form.
Turbo Tax does not support the gift tax form 709, but here is a link:
https://www.irs.gov/pub/irs-pdf/f709.pdf
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