There is no way to change resolution in-game and it's unplayable if your monitor is set above 1080p. It's genuinely much easier to play the original game than it is the EE on a modern setup, which is bizarre!
Baldur's Gate II has one of my favourite villains: Irenicus. Actually, I bought the game because of him, so take that as you will.
The EE is fine (I think. I haven't tried modding it). Also includes the original version, though a complaint was that now it costs more because it's packaged with the EE. One of the UI improvements I liked the most was telling you what the buttons do on hover.
Like the title says, the included game Baldur's Gate 2 Complete doesn't launch in macOS 10.14 Mojave. So far GOG's only stand on this has been that they don't provide support for that game.
So yeah... You're basically buying an unsupported game there.