There isn't a huge difference between the GOG and Steam versions of DD. Larian re-did the widescreen support and Win7 compatibility updates, but other issues remain (for example, some people have reported the same music issues, to various degrees).
Larian is looking at all the reported issues with both versions, and has stated they will release the latest version on GOG if there are no issues doing so. Presumably when there is a patch for the Steam / Larian Vault release they will look at bringing that updated version to GOG.
There is a Divinity game collection planned for October, promoted at Gamescom, but no details have been announced yet, AFAIK (see
Gamescom 2012). Larian hasn't confirmed any patch details for DD, but presumably they would want to get that done before the anthology goes gold, so hopefully there will be an update for the GOG version of DD before or around that release.