On my smartphone I cannot access any submenu items, because they are only visible and selectable after hovering the cursor over the main menu item. Since my smartphone has a touch screen, and thus has no cursor, I cannot access e.g. My Friends under My GPS

Touch ans hold, then it'll drop down and Stay. You may get phone options Menu up as well, just tap to the side and they will close.

Thanks Tony. I just realized that is more of a general problem, than specific for GP3S (stupid me...). I found out that the solution you suggest works for iOS, but not for Android. When using Firefox on Android it is even impossible to get it to work. When using the built in browser you can get it to work. First touch and hold the menu item. Then the sub menu appears but also a pop up on top of it, You can then (dummy) choose "select text" from the pop up menu. The pop up will then disappear and you can select the required subitem. Very clumsy. A redesign of the GP3S-GUI to be more touch screen friendly would be appreciated.