I assume you are downloading your rides as TCX file(s) from Golden Cheetah and manually uploading them to Garmin Connect. I did some Analysis regarding the TCX Format to investigate the root-cause of your problems:
- "activity Name": TCX Format does not support an activity name to be uploaded to Garmin Connect (A ride has an ID which is a time-stamp, that's it)
- "Elevation gain differences": TCX supports a tag "AltitudeMeters" (which for mee seems to be the right one) - but Garmin Connect ignores it when uploading a full file containing also track data (and still calculates from the that data) / (I also found partly a difference above 10% in my files - the difference get's less for me when de-activating the "Elevation correction" in Garmin Connect, but - there is a difference (and there is even a small difference when loading the same file (FIT) directly from my Edge, of if loading the file to GC -> Export to TCX and then load to Garmin Connect.
- "lower average Speed": Same as above TCX does not Support such field in it's Format to directly fill. GC exports the "Duration" in the TCX file (which Garmin then uses for AVG Speed Calculation). GC uses the "Time Riding" to calculate it's Avg. Speed.
@GC Team - not sure if this is worth changing / since probably any other Software does it different as well (and TCX is probably not only used to import into Garmin Connect)
- "work in GC and calories" - when checking my TCX test file GC actually uses the "Work (KJ)" (internal field "total_work") 1:1 to fill the calories field in TCX - not sure if this right or wrong (what I found intereresting is that the KJ value and CAL for me are not to far away)
@GC Team - since I am not sure what the Work KJ value is expressing related to Calories in TCX - no Suggestion how to fix this
@Kurt - I am actually not trying to Keep that data is Sync. I use Garmin Connection just as a "cloud storage" and for some links to friends (so data should be fine but I don't need exact same data in both Systems). GC is the tool for Analysis and it's a great one !
Hope that helps a little.