If I purchased Divinity 2 from Gamers Gate, can I activate it here on GOG? Dumb question, but I could do this with The Witcher....

No. It is possible to upgrade to the Developer's Cut version from the Larian Vault if you have the original PC release of D2:ED, the add-on FoV or DKS (see here), but there is no way to activate purchases elsewhere at GOG.

GOG is a wholly owned subsidiary of CD Projekt RED, the developer of the Witcher games.

I am starting a new game in Developers Edition. Do I run Ego Draconis or Flames of Vengeance at the starting screen? Thanks

You probably figured this out on your own by now, but Ego Draconis is the beginning of the game. Flames of Vengeance is the end section of the game, which would start you off at character level 35.

Ego Draconis was the original release of the game, and Flames of Vengeance an add-on, with a combined package called Dragon Knight Saga, which was later updated to the Developer's Cut (for the Divinity Anthology release). Originally the ability to start off in FoV was added because there were too many changes made from the original release to maintain saved game compatibility (you could install the add-on and continue with the same save, but not transfer a save from D2:ED to DKS / D2:DC), so that would let people play the add-on without having to replay the game (if they wanted to re-create their character rather than try a different build or check out the remastered stuff).

thanks Raze