Can I file my 2020 W-2G with my 2021 taxes?

Sorry no.  you have to report the income the year it's for.

To do a prior year return you have to buy the Desktop program here,

https://turbotax.intuit.com/personal-taxes/past-years-products/

You will need a full Windows or Mac to install it on.

If you have a simple return and want to file for free you can fill out the forms by hand.  Here are some basic forms.....

Here is the 2020 1040 return

https://www.irs.gov/pub/irs-prior/f1040--2020.pdf

or if you want bigger type use 1040SR for Seniors,

https://www.irs.gov/pub/irs-prior/f1040s--2020.pdf

2020 1040 Instructions

https://www.irs.gov/pub/irs-prior/i1040gi--2020.pdf

2020 EIC and Tax Tables

https://www.irs.gov/pub/irs-prior/i1040tt--2020.pdf

Schedules 1-3

https://www.irs.gov/pub/irs-prior/f1040s1--2020.pdf

https://www.irs.gov/pub/irs-prior/f1040s2--2020.pdf

https://www.irs.gov/pub/irs-prior/f1040s3--2020.pdf

Don’t forget your state.  And you will have to print and mail your returns.  So be sure to attach copies of your W2s and any 1099s that have withholding on them.  You have to mail federal and state in separate envelopes because they go to different places.  Get a tracking number from the post office when you mail them for proof of filing.

Then when you file for 2021 you need to enter 0 for the 2020 AGI since you are filing late.