If I understand correctly, this is not FGear question but rather, a vehicle games/sim question.
It is not that hard, you need to use at least two images - one is for the background, and the second is for needle. The needle (an image widget) can be rotated and you can use “float curve” to achieve the mapping ie what widget angle = 0kmh for an example.
Build the float curve table and you will achieve it. If you need more information, you can ask…