If you have forgotten your login account details, you can search in your email inbox to find the account creation notification.
Your login details are:
The authentication method, which is Salesforce Production/Developer, Salesforce Sandbox, LinkedIn, Google, or an email address and password.
Your username, which may or may not be your email address.
If you signed up with an email address and password, your login account is that email address.
You should have this information in an email like this when you created the account.
You can use this information to log into the Gearset app at https://app.gearset.com/.