Gear ratio is fine (really all these measures are fine!), but it depends on what you're trying to accomplish. If you're just trying to see if the distribution of gears is even and what the best gear shifting progression is, any measure should work as long as you have criteria for deciding what's good. If you're trying to estimate leg effort (pedal force, power, aerobic load, or ??), fitting crank length into the mix is worthwhile. But if you don't think an objective analysis of effort is meaningful, which I don't, then spread is what's important, and effort is judged subjectively. That's what I mean when I say gain ratios or gear ratios are not useful for me.
Road Fan