I've heard some gossip... can anyone confirm if official free plugins like Entertainers or BCSounds are included in the GOG version of Morrowind GOTY?

Yes, they're included, but not enabled by default.

(When the launcher starts, go to "Data Files", then enable whichever plug-ins you want. You can also disable Tribunal and Bloodmoon, if you'd rather not play with them (some players don't like having the assassins appear early in the game, and hence might want to start out with Tribunal disabled, for example).)

Rather than disabling the entire Tribunal and Bloodmoon expansions, it's better to just add a small mod to delay the assassin's until an appropriate point in the game, like this mod here- https://www.nexusmods.com/morrowind/mods/33973

It's definitely a nice touch to have all of the official addons pre-installed. I was quite happy when I went to install all of them, and saw they were already all present. The GOG version is definitely the better version to get. It has a few extra touches of polish, that other versions don't come with. I've also noticed that the GOG version has all of the game assets extracted from the main archives as well, and placed into their proper folders, which I suspect was done for faster load times, eliminating texture pop-in, and it should reduce or eliminate stuttering between loading cells. It's yet one more very nice finishing touch from GOG. Overall, it should provide a smoother game experience, and should be an especially noticeable improvement if you're using a traditional hard drive, have an older system, or are using a modest laptop.

The GOG version also comes with a scanned copy of the Prima game guide, which I don't believe the Steam version comes with. I already had a physical copy of the Prima guide, but I found it absolutely indispensable for finding every last bit of content, when I got the guide many years ago.

Besides all of the extra TLC treatment that GOG usually gives to their games, there's one other reason to consider GOG, when deciding where to buy any older game. As time goes on, GOG continues updating their games to ensure they continue to run on modern systems. Older software on Steam almost never gets updated. If a newer version of Windows breaks an older game on Steam, you're usually at the mercy of hoping for a community patch, in most cases. This probably won't be a major problem for Morrowind, with OpenMW available as an alternative engine, but it's still something worth considering for any older game.

Also Arena+Daggerfall as a bonus, eh