the GPS does not come into play on a cross trainer. It comes down to the measurements Apple uses vs what the machine is using. I has a similar issue on the treadmill with a Fitbit. The way I solved the problem was to take my fitbit watch and into lace it on my shoe. So i counted / tracked more accurately. Unfortunately the AW requires turning off wrist detection, disabling ApplePay and the passcode feature in order for attaching the watch to you shoe lace to work.
It would be smart if if someone made a companion device similar to what Nike had for your show and bluetooth relay it to your watch or health app.