I don't think there is an option to show only on desktop, cause this app sits on top of every app.
It also depends on the image you are using for the startmenu. Try changing the image.
Yes, that should be hideleft, cause right will show the menu.
I don't use the hide feature coz, I use a small image and placed it on 0,0. i.e top left. so the startmenu covers only half of the gnb app launcher menu.
The windows mobile theme shows the "Start" image as the launcher menu and I've set the same image as startmenu image.
So it does not show as an extra image on the screen.
