If Simbrief is not connected: The disconnection to Simbrief is shown by a red button "Reload Simbrief" (at the end of the menu). I have the feeling you cannot control it bei the FMC of the PMDG airplane.
I fail to see what's the connection between GSX Simbrief integration and the PMDG FMC.
Were you expecting GSX sending the Simbrief flight plan to the FMC ? Nowhere the manual says the GSX Simbrief integration is supposed to do that, which will a fairly complex feature with almost no benefit, considering the most popular airliners like the Fenix or the FBW A320 already have their own Simbrief integration, and PMDG will soon have its own EFB doing that.