Like most things generic, most aftermarket gauge sets come with fuel gauges that have the ohm rating for a GM fuel sending unit.

You basically have 2 options.
1) Get and install a GM fuel sending unit in your tank.
2) Buy a fuel gauge that matches the ohm rating on your existing sending unit. Before you go this way, be sure your sending unit functions correctly and is in good condition.

I would be suspect to the adaptor Speedway offers. I think I would rather just install a GM style sender in my tank then put something in between the gauge and the sender that is suppose to fix every ohm rating difference. Speedway has not been known for having high quality stuff.

It may be easier and cheaper to just install a free standing fuel sending unit for the GM ohm rating then any other option. It could be as easy as simply installing a new GM style sending unit into your tank, I believe there are several free sending units on the market. Gene