Roll down tests are great for testing tyres on relatively even tarmac, but its a pretty poor test for a tyre designed to be used on rough, loose surfaces.
The whole subject of ‘what is the fastest gravel tyre’ will be completely dependent on the type of riding you do - what sort of gravel and in what sort of conditions. Unless you’re talking about really smooth light hard packed gravel like on a fire road, you could end up losing more time struggling with grip on rougher sections, climbs and corners than you could make up with a super low rolling resistance tyre on flat smooth straight sections. Grip when putting down power, especially on sketchier surfaces, is much more useful to me for gravel riding than outright speed on a good surface (because of the types of gravel I end up riding on).
If you want to do a test, try to find a short local gravel loop that is representative of the riding you do. Do a load of repeats of the circuit, trying to keep your power even. Swap tyres and repeat. It won’t be anywhere near as scientific as a roll down test, but at least the data that it provides will be relevant to the question being asked