No way that I'm aware of to remove the GPS information from your existing activities.
The devices I own have an option to not record the information in the first place: use indoors for forerunner, disable GPS for edge.
But then unless you've got a cadence / speed sensor or similar you basically just bought a very expensive heart rate monitor :P
Don't get me wrong. I want to track this data. But I don't want to upoad this. It is enough for me to see the gps-tracking-data (for example with trainings center) on my locale computer. On the other hand I like the reporting possibilities in garmin connect for distance, times , between times etc. The only thing I don't want is to see online my gps-tracking.
I don't think this is easily possible, and it's definitely not supported. In theory you could convert the device's .FIT activity into a TCX then set all instances of <LatitudeDegrees> and <LongitudeDegrees> values to a dummy value like the north pole.
I'm not sure how GC would behave with that data though, I'm assuming it actually uses the builtin speed and time tags rather than generating statistics purely from GPS data. If I'm wrong then you would have just removed all the information GC uses to generate your speed information.
