Twitter dev support has not gotten back to me about restoring the connection between GSC and Twitter for login, so we can consider the door closed on that for now. 
If you used Twitter
You need another login method that uses the same email address. As of v1.7.0 of Granny Squares, you can simply choose to register with any other option on the login page if the email address matches your Twitter email.
You will be sent an email to confirm that you meant to add an authentication method to your account. Just verify it, and you should be good to go.
If you happen to still be logged in, go to the new Profile page and use the “Add a Login” section to link with another method.
If you run into problems
Just contact me and I’ll help. There honestly aren’t a ton of you who used twitter, so even if most of you need manual intervention, we’ll get through it. 
The most likely scenario I imagine is that you register with a new method and it turns out the email addresses didn’t match, so you don’t see your blankets. Reach out, and I’ll do my best to verify you are the owner of both accounts, and get the accounts linked up manually.