Divinity 2: Developer's Cut will not finish downloading. It is stuck at 5.89 GB. I am using GOG.com Downloader. I left it to download overnight yesterday and noticed it did this. So i deleted it and tried it again.
What am I missing or doing wrong?

Did you try pausing and resuming the download, or exiting the downloader and restarting?

Do you have an anti-virus program, or something, running that might be interfering with the downloader finishing off the download?

Well I just closed it out and just went to the install to see if it would. Although it said it was not done downloading it installed okay.

Mine did that when I was downloading to my flash card. It left one MB to download but the installer came up fine ( I did not try to install however). So if I did install it somewhere makes me wonder if something might be left out of the game? I think it was Planescape Torment that did this also....

If the integrity check the installer does when it is launched passed, then it is likely the download itself finished and the downloader just had a problem finalizing whatever it was doing, so the display didn't update.

However, if you want to make sure, there is a GOG Community Wiki which lists the md5 values for many of the games, and you can use a program like Hash to calculate checksum values for the files you downloaded.