Note that you must let your arm move during the indoor walk so the watch can measure your movements. Steps and stride length. No GPS is used.
GPS should always be turned off indoors and is by default turned off for all indoor activity types. That means that you can't use an outdoor activity type indoor without turning off the GPS. If you keep it on you will get very strange results.
If your watch got a GPS, it will never use the GPS in your phone.
