I bought the game from Steam. Until now, any updates have been installed automatically. However Gift Bag 3 has not been installed. The install button in Steam is lit up but it wants to install 59,000MB which must be the whole game which is already installed.

Please can someone tell me if it's possible for me to install Gift Bag 3 manually and if so, link me?

Thanks,
LS.

In Windows Explorer, try browsing to the '..\SteamApps\common\Divinity Original Sin 2' folder, and check if the install files are still there.
If it is just a problem with Steam (if the appmanifest_435150.acf file in the SteamApps folder is deleted or corrupt, that will cause Steam to list D:OS 2 as not installed), re-installing should get Steam to check the folder first, usually recognise the files are, and fix it without having to redownload everything (though sometimes it still does).

Thanks , I've got it now.