Simple Pinch and Pan offers a free and easy to use way to implement pinch zoom and panning into your mobile game.
You can have an object-following or free camera and fully customizable options for zooming speed, max and minimum zoom, and can lock the camera when you need to.
Usage is simple; just place the controller object and set its properties at its Create event. You can dynamically enable or disable object following, change the object being followed or edit any settings to create the perfect view for your game.
Make sure your set your target platform (android, etc) graphics scaling settings to "Full scale" in the Global Game Settings menu.