I'm on Ubuntu (16.10), actually Kubuntu.
When I do ./starts.sh, I only get


Running The Book of Unwritten Tales 2
launching "/home/mick/GOG Games/The Book of Unwritten Tales 2/game/launcher/1.0.0/Launcher" with arg(s): ()
/home/mick/GOG Games/The Book of Unwritten Tales 2/game/launcher/1.0.0/Launcher: error while loading shared libraries: libgstreamer-0.10.so.0: cannot open shared object file: No such file or directory

Well, it's true, gstreamer 0.10 was removed in yakkety. It's not in debian testing either. gstreamer 1.0 dates from 2012 and 0.10 stopped being maintained in 2013.

Now, how do I launch bout2 ? :D

I also send a support request to GoG, but it seems nobody reads them, so I don't see how I'll work around this.
Let's just hope there aren't too many games like that in my library.

OK,
I did the actual support work. I may as well share it as as soon as yakkety is released, all non-LTS users who upgrade will be impacted. And I just checked, my debian sid computer would be too.

The link failure is not in the game executable but i the launcher executable (named 'launcher').
You can start the game by finding the actual game executable (BouT2.x86_64 for me not sure if it always the case). A ldd run confirms that my system libraries allow to link the game, no need to fiddle with LD_LIBRARY_PATH (but your mileage may vary).
So I can just run ./BouT2.x86_64 (in game/Linux direcory) and the game starts just fine. Well I don't know if everything will run fine, but it starts.

I hit the same issue in Ubuntu 17.04. gstreamer 0.10 is no longer available, only 1.0 is. The used library should be bundled with the installer. Installing the old version is not very practical.

I did manage to get the launcher to work by manually installing the old gstreamer libraries from 16.04

This can be done by downloading these 3 packages. I can't post links yet, but you can find them if you google for "ubuntu package database", head to the first link, under search package dirrectories, choose xenial as distribution, and search for them.
libgstreamer0.10-0
libgstreamer-plugins-base0.10-0
gstreamer0.10-plugins-base

After downloading them, to install, run for each file, in order:
sudo dpkg -i filename.deb