Click on the Shortcuts icon, and long-hold one of the blank tiles. It will prompt you to add a shortcut (browse to the GMPC exe). It will ask if DriveLine should disconnect from the GPS when running that app. Select YES unless you are using a port-splitter for your GPS comm port.
Another option is to create the shortcut within DriveLine again, but this time, when it asks if you want to "leave app on desktop", select YES. This will allow GMPC to run full-screen, and DriveLine will display as just small floating interface on the right-hand edge of the screen.