The way I understand it, gas gauge senders use a rheostat (or something of that sort) that changes resistance with the level of the fuel. This should clear up any confusion on this issue:
http://www.howstuffworks.com/fuel-gauge.htm According to this, the gauge measures the current, albeit in a rather unusual manner.
Different automakers use different ranges of resistance in their senders, making the fuel gauge mix-and-match nearly impossible. They don't even read the resistance in the same direction, with some manufactures low resistance is full, others use low resistance as empty.
