This is a simple digital image processing task. I am proposing a quite efficient solution without any error correction mechanism, so this is not a production quality approach. But as a first proof of concept it should do the trick.
The easiest approach would be splitting the image in two channels - one for the color red and one for the color black. You can neglect the rest outside a certain threshold. If the lighting is good, this should also deal with the background.
Step 1: Crop away everything outside the gauge to simplify your life. The easiest way to do this is just putting the camera close enough that it only captures the gauge. That way you don't have to implement it in your algorithm.
Step 2: The center of your image will be the red dot in the middle of the gauge. You can easily identify it with a filter looking for the largest concentration of red in the red channel of your image. Calculate the center and remember the coordinates.
Step 3: Do the same for the largest concentration of black in the black channel. This will be the thick end part of the black needle. Calculate the center as well and remember the coordinates.
Step 4: Calculate the vector between those two coordinates. The angle of the vector will tell you the angle of the needle (take care of the direction of the vector). Once you have the angle of the needle, you can easily derive the number it is pointing to.
There are obviously much more elegant approaches to solve this problem, but this quick and dirty algorithm should do the trick without a lot of hassle. You need to make sure that the camera is positioned in the right angle, because we haven't implemented any error corrections in our algorithm. You also need to make sure the lighting is sufficient and that there aren't any other significant influences on the image quality. As long as the orientation is stable and the image quality high, this should enable you to read the gauge with good precision.
