Due to a recent update to CS:GO, clients must provide a steam account token to the leased game server in order for it to function. This is an attempt by steam to curb illegal modifications.
Everything is automated, and the process is generally simple.
After purchasing a server, you log into server administration and see the following table

If you already have a GSLT token, just add it, if not, click "here" for instructions on how to create one.
