1. Something doesn't work
1. I'm getting this error all the time: No camera layers. Adding one for the main camera.
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.
2. TUIO/Windows 7/iOS (multi)touch doesn't work.
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.
2. Touch input
1. Is it possible to test multitouch gestures without a multitouch device?
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.
2. Does TouchScript work with GUI.Button?
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.
3. Working with gestures
1. How do I make gestures work together?
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.