If you have forgotten your login details, you can search in your email inbox for the account creation notification.
Your login details includes the
Authentication provider - this could be
Salesforce Production/Developer,
Salesforce Sandbox,
LinkedIn, or
Google
Your username - this may or may not be your email address.
You should have this information in an email like this when you created the account.
You can use this information to try to log into the Gearset app at https://app.gearset.com/.