Here is the solution to be able to download music to your Samsung Galaxy Watch or Gear S3 and the Samsung Gear watches with Spotify, to enable you to be able to listen to music from your watch offline when you are away from wifi or bluetooth (out running, at gym etc)
Many people have had issues with being able to download music to their watch to enable them to have offline listening.
Here is the solution...
1. Your mobile phone will likely be connected to your watch but to do the download process on your watch you must allow your watch to standalone for this process so I suggest you put your phone into Airplane mode to ensure it is temporarily disconnected from your phone.
2. Once you have disconnected the connection between watch and phone, make sure that your watch is connected to wifi by going to the settings area on your watch (not the Spotify app) and select connections, wifi and connect to wifi and for the moment select always on.
3. Open your Spotify app on your watch, go to settings and select playback, stream on wifi. You will be asked to connect to Spotify via your Spotify username or Facebook credentials.
4. Once you have logged in to your account, go to 'Your Music' on the Spotify watch app and this will show any of your music previously added to your Spotify account, such as your collection or playlists. As an example, if you wish to download to your watch a Playlist, select the playlist you wish to download and it will show a 'Download' option. Select Download and the music will start downloading to your watch.
5. If you wish to listen to your downloaded music offline when you are out at the gym, out running etc then on your Spotify app on your watch, select settings, offline, go offline, and this will then allow offline playback without needing a connection to wifi or Bluetooth.
I hope this helps anyone who had the same painful initial issues that I had :-)