The GoG store page for Baldur's Gate: Enhanced Edition appears to suggest it inclues the expansion "Tales of the Sword Coast," but I don't see a way to start a Tales of the Sword Coast game in the game menus for the Enhanced Edition of Baldur's Gate.

I tried right clicking on the game and I think configure install or whatever it is you do to select the available DLC to install for the game, but I didn't see any DLC available there to mark to install. And I don't see Tales of the Sword Coast as a separate game anywhere in my GoG game library.

So how do I start up Tales of the Sword Coast?

Go to Ulgoth's Beard. There is a quest for Durlag's Tower and another one for an expedition. I think the dwarf at the inn would direct you to both. Those two quests are the Sword Coast expansion. Head to the southwest corner of Ulgoth's Beard for the expedition quest. I think you can hoof it straight Durlag's Tower instead of going to Ulgoth's Beard for that part of the x-pac.

Edit: I forgot the third quest-line of TotSC. There's a mage who will ask you to retrieve an item. He will teleport you to an icy area. He's in the middle of town IIRC.

understand that its optional and designed for high level experienced parties

Am I supposed to do all this BEFORE killing the final boss from Baldur's Gate? I had thought the expansion was something you play after you finish the game, but after the final fight it appears there is no way to continue on to the expansion using the character I just won the game with. Should I load a game from before the final boss fight and go do Tales of the Sword Coast on that save file?

Also, how good is Tales of the Sword Coast? Does it have a campaign/story worth playing?

And do I need to know the Tales of the Sword Coast story before going on to Baldur's Gate 2?

Yes you'll need to reload from an earlier save.
TotSC has some of the best quests in the whole of BG1. Don't be discouraged by a fairly forgettable dungeon romp one at the very beginning tho. The other two (huge ones) are the sh*t.
Story irrelevant for BG2.

As alluded back in January, Tales of the Sword Coast is just some extra areas reachable while playing Baldur's Gate: Enhanced Edition. It is part of the main campaign, not a separately installed or separately triggered campaign. If you were not specifically aware that these areas were added in the expansion, you might play them without realizing you were in areas that were not part of the original retail release of Baldur's Gate. (Though when you finish all of the TotSC quests, you get a journal entry that remarks on this.) Defeating Sarevok ends the entire campaign, preventing you from playing any other areas, both original optional quests and Tales of the Sword Coast quests.

In contrast, Beamdog's expansion "The Black Pits" is installed by default with the Enhanced Edition, but is a completely separate campaign, accessible only from the main menu. You can export characters in/out of it, but you can't reach it by traveling the world map in the main campaign.

Similarly, in Baldur's Gate 2: Enhanced Edition, The Black Pits 2 is a separate campaign. Baldur's Gate 2: Throne of Bhaal is in an odd middle state. It can be accessed from the main menu of Baldur's Gate 2: Enhanced Edition, but it is also started automatically by a party that completes the Shadows of Amn campaign in BG2: EE. Further confusing it, Throne of Bhaal in BG2: EE has a companion quest that takes you to an area that is clearly copied from Black Pits 2, but works differently in a few ways.

Actually, I didn't find Ice Island all that boring; I assume that's the "forgettable" dungeon referred to earlier. At the very least, the first fight against three mages at once is fairly challenging.

Don't forget that Watcher's Keep (originally added in Throne of Bhaal) can be accessed in Shadows of Amn or Throne of Bhaal. (Just make sure you're of a level that handle it, of course.)

Didn't say it was a boring one, only just it's more conventional (both in terms of size and composition) compared to the two main quests. Relatively challenging, yes. But hardly more than that.

Neither TotSC nor ToB need any activation or separate gameplay.

They are both integrated in the gameworld. You just have to visit the additional content by yourself.

Obviously, this wasn't so when they were released. TotSC at least.

Why yes obviously.

When you have already the game finished and an expansion comes out, you want to be able to continue playing.

But the EEs come with the expansions included, and even if you activate the classic version here on GoG you also get the expansions already included, not as a separate install.