I agree with '6 there, you're need to connect the purple/black to earth (brown) rather than a +ve supply. If you connect that purple/black to a +ve supply, you may well do damage to your gauge and or associated components.
As '6 says the sender unit is a variable resistor, so another thing you may want to try is connecting an ohmeter between the two connections on the sender unit itself and measuring the resistance of the variable resistor. Then after you have used some fuel, try measuring it again. I would imagine that with a full tank you will have high resistance and with an empty tank, low resistance.........although knowing my luck it'll be the other way round.........either way you should see a change in resistance as you use the fuel.