1. Something doesn't work or you are getting errors
2. Touch input
3. Working with gestures

This is not an error. It's just a warning that you don't have any camera layers in the scene and one was created automatically.

Make sure that you have an appropriate input source in the scene. To receive mouse and touch input you need to have a StandardInput in your scene. To receive TUIO messages you need TUIOInput. Note that Windows touch in the Editor is supported only starting from Unity 5.6.

Yes, you can use StandardInput for that. Just press ALT and click anywhere to generate a fake touch which will be active and stationary while ALT is pressed. Read more here.

The short answer is "sort of". StandardLayer works with objects which have colliders on them. GUI is not a part of 3d scene and GUI objects don't have colliders. But it is possible to implement a custom touch layer with all the functionality needed for GUI buttons.

For gestures to be recognized simultaneously you need to put one into another one's Friendly Gestures list in inspector or call gesture1.AddFriendlyGesture(gesture2); from a script.
Otherwise the gesture to begin first takes all the touch points.