The GNOME built-in apps seem to work just fine in touchscreen mode. Yes the buttons and window titlebar are larger, but not too much. (And there are ongoing 'tweaks' that allow for a better use of the resulting space; e.g. the large, touch-usable titlebar now has a room for a subtitle and a few app-specific buttons.) The clearest issue with GNOME on touchscreens right now is that it ships with a crappy on-screen keyboard, missing many of the PC keys (which leads to usability issues with many apps) and a lot less usable than what you can get on AOSP.