Note: for me, I added the "spotify music" application near the top of the auto-generated list that Nvidia control panel gives you when you click add. No .exe file was visible under users/*my name*/AppData/Roaming/Spotify.
This "fix" did disable G-Sync completely for me. And not only when Spotify was running, but also in general for all games etc. After I did re-activate G-Sync (as described in an earlier post) the problem with Spotify was fixed without any additional configuration in the Nvidia control panel.