If you have a modern car with a manual transmission, it probably has a digital display of what gear you are in. This is done by math. The computer knows your speed, the wheel circumference, the rpm, and the various gear ratios available. So it does some calcs and tells you what gear you are in.
This can also be done with bicycles, both in real time and calculated afterwards if one has a cadence sensor being logged by RWGPS.
I did so by exporting a ride in GPX format, and importing it into a Filemaker database. I calculated and entered my gear ratios and the rolling circumference of the tire. From that I was able to display what gear I was in at what point or time in the ride.
It's not perfect, especially for MTBs, as the tire circumference is more subject to change based on laden weight and tire pressure. It would be more reliable for road bikes. When you are not pedaling no ratio can be calculated but one can assume the ratio to be the last used until one starts pedaling again. And the ratios only somewhat match up due to many changing variables, so you have to have windows (e.g.
Gear Ratio > .1 and Gear Ratio ≤ .75 ; "Gear 1" ;
Gear Ratio > .75 and Gear Ratio ≤ .85 ; "Gear 2" ;
Gear Ratio > .85 and Gear Ratio ≤ .87 ; "Gear 3" ;
Gear Ratio > .87 and Gear Ratio ≤ 1.04 ; "Gear 4" ;
At any rate, being as hungry for ride data as I am, it would be nice to see this displayed along with grade, speed, etc. And even more so, it would be great to have a real-time gear display in RWGPS. One simply has to enter the available gearing and measure and enter the rolling distance.
Oh, yes. Having more than one front chainring does complicate things. I think that road bikes, which as explained above, could have small enough windows to reasonably display which combination of gears are being used, and many MTBs only have a single chainring.