I hope this is a good place to post this question ... I experience issues with uploading manual TCX exports to Garmin Connect in both v3.4 and in v3.5-DEV1710 and v3.5-DEV1802_64bit_Windows.exe. The issue under 3.4 was that activity distance and time would not appear in Garmin Connect requiring me to manually edit that data and then estimated calories would not calculate.
In the two development versions both TCX and FIT format exports produce an error when manually importing to Garmin Connect (which displays: An error occurred with your upload. Please try again.). I've found that if I remove the <Creator> section of the TCX file, the edited file will upload and be accepted by Garmin Connect. I am unable to parse the FIT file so am not sure where that error is. (I should also note that I dont usually record activities on Garmin devices - ie activities are recorded on my phone or via Zwift etc so Golden Cheetah becomes a vital step in collecting and processing activity data).
So right now, I would not say Manual upload works 'out of the box'... editing is required. If there is an alternate workaround for this issue or its a known issue (I did not see the problem identified on the development open issue list) please let me know.
For those opposed to manual uploads, there may be a way to get data to Garmin Connect via Fitnesssyncer (which integrates with Google Drive, Drop Box, Garmin Connect, and others ) They appear to license the full Garmin API and provide they only way I know to export Wellness data from Garmin Connect. I've used Fitnesssyncer to extract data from Garmin Connect not import data so cannot comment on whether it could be used as a platform to bridge data from Golden Cheetah to Garmin Connect).