Ancient-Red-Dragon: If you buy any game on GOG then you get it
instantly, immediately after the checkout process has been completed. There is no wait.
Technically, you don't have the game until you've finished downloading it. Until you start a download, there can't be an estimation for the delivery. Usually there isn't a question of delivery "date", but more commonly it's "time", seeing how the delivery is through the internet rather than the postal service. But I suppose if you were to download The Wither 3 over 28.8kbit/s modem, it'd take a while, so "date" might be appropriate there.
EDIT: I don't have The Witcher 3 on my GOG account, so let's use The Witcher 2 as an example.
4 files at "3.9 GB"
1 file at "2.2 GB"
1 file at "1 MB"
I'll assume 1024 MB per GB, but GOG might count 1000.
Counting only the GB+ files makes 17.8 GB. Multiplying by 1024 and adding the 1 MB of the installer executable makes 18228.2 MB, multiplying again by 1024 makes 18665676.8 kB, multiplying by 8 to make kb gets us 149325414.4. Now, calculating the time from that. Dividing by 28.8 kb/s makes 648113.8 seconds, 10801.8 minutes, 180 hours, or 7.5 days.
So, yeah, anything at about 2.5 GB or more would be over a day (again, given a 28.8 kb/s modem - you really should upgrade to a 56.6 kb/s one, I don't know how much v90 would help given that the data can't really be compressed more than it already is, but any modem you'll find today probably has that anyway). With a ~20GB game, yeah, the estimation of when it'll have arrived would be about a week after "delivery" started, so a date estimate is quite appropriate.