Yes, I have Silent Storm Gold Edition on both GOG and Steam, and I can confirm that the Steam version of Sentinels can be installed and played separately from the original game.
By the way, you don't even need to have it on Steam to make sure that Sentinels is a separate game there. You can use
SteamDB to see that Sentinels has its own depots and doesn't use any depots from the original game.
It has always been a standalone add-on since the initial release. I have no idea why GOG decided to merge it with the original game into one library entry.
It would be nice if GOG finally will gave us the ability to install Sentinels separately, as originally intended by developers.