Referring to your image, the pink wire runs to the fuel gauge, the pink/white to the left fuel tank, and the pink/black to the right tank.
While the ignition is switched ON: temporarily connect the pink wire to a clean ground - the dash gauge should register empty. With the connector unplugged as you've pictured it, the gauge should read well past full at about the 3 o'clock position. Jumper the pink to pink/white and the gauge should indicate the fuel level of the left (driver side) tank.
The green wires in your image are power and ground to the selector valve. Which green is power and which green is ground depends on the position of the dash switch.
If the selector valve is still installed and plumbed, as long as the valve is fully set to the left tank, whether the right tank is physically there or not is immaterial. But, since the right tank is "gone," if you're not planning on replacing it, remove the selector valve too and plumb the lines directly to the left tank. Then jumper pink to pink/white and (assuming your dash gauge and tank unit are okay) your gauge should function correctly.
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