When setting up GLS as a carrier, you need to provide the following:
Username and password are whatever you use to login to YourGLS with.
ContactID however, is a bit harder to find at first glance.
When logged into YourGLS, select Administration > User Settings.
On the right side of the screen, you should see two numbers listed as "Shipper number" with the latter being your ContactID.
Your CustomerID is the same as your login, but is also displayed in the first half of the "Shipper Number" if needed.