Not sure exactly what your question is, but if it's about compatibility with the etrex touch, then likely the Garmin webpage is correct, though they've goofed on occasion. The reason the manual says gsc10 is that at the time the etrex was released, only the gsc10 was available or the software only supported it. If it was due to the device software, then likely a later update to the etrex's software made the newer cadence sensor compatible.
As for the cadence only part... some Garmin devices will not handle the speed information that comes from the device as the actual speed itself is not passed in the data from the sensor, and the speed must be calculated from the data provided. Some of Garmin's devices were not intended for this type information and were not programmed to do the calculations and display them. I believe there are some devices that will not use the cadence info, but will use the speed info from a gsc10.
