You need to contact your Access Administrator (AA) to get access to GCSS. They will send you the link to self-register. That must be done only if you are in a bucket position. A bucket position means you haven’t been given roles.
After waiting for 24 hours, the AA will transfer you into an actual position.
What if you don’t know your AA? You have to contact the GCSS Army help desk. Don’t forget to provide your UIC as well. They will help you by researching your unit structure. That way, who your AA is can be determined.