GCP Marketplace license
GCP Marketplace License Overview
Customer can purchase GitLab subscription from GCP Marketplace. This purchase allows the customer to pay for the subscription monthly.
Even though, the customer is paying for the subscription monthly, they signed an annual or multi-year contract with monthly payments. So they cannot reduce the number of User mid-term. They can only reduce at renewal time - which would mean churn.
The customer purchased via GCP counted as reseller customer so they would not have access to the customer portal and there is no way to add more seats from GCP either, see GitLab issue 4930. To make change to the subscription, the customer would need to reach out to Sales team.