Sounds like you want to disconnect the fans from the GPU and hook them up to a front panel? Not sure I have seen anything that does that. Agreed, software control is much simpler for this.
Most GPU fan connectors I have seen are the 4/5/6 pin connector that is quite a bit smaller then normal Case or CPU fan headers. (Number of pins depends on the number of fans, up to 3)
Occasionally, but very rarely, the manufacturers spec sheet will give you the pinout on the fans. More often the pins will be labeled directly on the PCB. If not, I would install the GPU normally and measure the voltage while the fans spin.