The GameMaker Account is a one-stop login for all GameMaker services such as GameMaker licensing/ subscriptions (through Steam or the Desktop version) as well as the GameMaker Marketplace.
You can find out more about GameMaker Accounts here: Creating A GameMaker Account
Tip: If you believe you have an account but are unsure then please try creating an account with your email address, it will then notify you if the email address is in use.