Level Mode
In response to Tom's comment, you can wire a discrete input to the GEA 24 and configure it for a level mode input.
I do not have GMC 305 or 307 in my G3x touch panel. I do not find myself wishing for one while flying. I have about 150 hrs on this configuration with quite a bit of IFR and cross countries. That being said I can definitely appreciate the benefits of the GMC. There really isn't room in my panel for one which is what drove my decision. I do have a second display which acts as a back controller.
To answer an above question, all functions of the GMC 305 including level mode show up on the pop up AFCS window, but it does require two touch screen button presses versus one key press.
In summary, If you have the room and funds, you would never regret putting in a GMC. The 307 is the best GMC for G3X. If you for some reason you can't put one in (cost or fit), you can absolutely operate the airplane effectively without one.