If you are not someone else's dependent for tax year 2020 then when you file a 2020 tax return you should be able to get the stimulus money.  That money was an advance on a credit that can be claimed on a 2020 tax return.  But make sure your GF does not mess up and leave you on her next tax return and claim you again.

".....I started receiving SSI..."  

Are you receiving SSI  (Supplemental Security Income)  or Social Security benefits?  They are not the same thing.  Be clear on what you are getting--when you ask questions about it, it is important for us to understand which kind of income you have.  If you receive SSI you will not receive any sort of tax document for it in January.   If you are receiving Social Security benefits you will receive a SSA1099 in January.

WHAT IS THE DIFFERENCE BETWEEN SOCIAL SECURITY BENEFITS AND SSI?

https://www.ssa.gov/ssi/text-over-ussi.htm

Sorry for not clarifying. I receive Supplemental Security Income. Can I or do I still fill out a tax return? My gf won't be putting me down as a dependant for this year. Thanks for the advice.

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@YearJ wrote:

Sorry for not clarifying. I receive Supplemental Security Income. Can I or do I still fill out a tax return? My gf won't be putting me down as a dependant for this year. Thanks for the advice.

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SSI is not reported on a tax return.  If that is the only income that you receive then there is no reason or need to file a tax return.

Ok, I see. So I sit back and wait to see if I get the stimulus?