It depends. Here are the criteria she needs to meet to be a qualifying dependent.
The person can't be your qualifying child or the qualifying child of any other taxpayer.
The person either must live with you all year as a member of your household (and your relationship must not violate local law).
The person's gross income for the year must be less than $4,300.
You must provide more than half of the person's total support for the year.
If she meets the criteria above, you can add her to your return and answer the interview questions that will indicate she meets this criteria in the "My Info" section.
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If your GF had less than $4300 of income in 2020 --- not counting Social Security--- and she lived with you the whole year, you can claim her as a qualified relative. You will get the $500 credit for other dependents for claiming her, and the 3rd stimulus check of $1400. You do not get the first or second stimulus check for claiming her.
CREDIT FOR OTHER DEPENDENTS