Why isn't this Account (GL Code) available?
Syncing should happen on a pretty frequent basis so if it has been more than 15 minutes since adding the Account in GP, the most common problem is that Purchasing is not selected on the account under the Include in Lookup section.
To correct this, you'll need to open up the Account Maintenance window by navigating to:
Cards > Financial > Account
Then look up the account in question and ensure Purchasing is selected in the Include in Lookup options.