For using GameObject buttons with the Gunman/Hitman asset packs, I strongly suggest using the InControl Unity Package.
Each input scripts contains optional compiler directives for InControl and Rewired control schemes.
#if IN_CONTROL
using InControl;
#elif REWIRED
using Rewired;
#endif
Define either InControl or Rewired globally using the Scripting Defines field in Unity's Player Settings, or define it at the top of each Input script (ie: GunmanInput.cs)
This method is best because you can seamlessly switch between On Screen controls and Bluetooth Gamepad controls.
InControl in particular has excellent support for on-screen buttons for Mobile games. It is what I use myself! 🙂