bobsobol: May sound daft but... I'm having the same issue here. How would I do that?
Some research on askubuntu suggests:-
sudo dpkg --add-architecture i386
sudo apt-get update
sudo apt-get install whatever whatever whatever
or, just:-
sudo apt-get install whatever:i386 whatever:i386 whatever:i386
But I'm having trouble pinning down exactly what 32bit packages those libraries are part of. (ie. what "whatever"s I need to install)
For the record, I'm on Ubuntu 14.04 LTS. (with xfce4 rather than Unity, aka Xubuntu 14.04)
--- EDIT ---
Okay, I found my answer on a Steam forum. :(
WMP suggested the following, and it worked for me:-
sudo apt-get install libsdl2-2.0-0:i386 libsdl2-image-2.0-0:i386 libcurl3-gnutls:i386
I would think (for the GOG packagers) this confusion could be avoided by adding this line inside the postinst.sh on "if" condition that uname -p results in x86_64 rather than i686. ;)
As a warning to those using other Linux distributions (presumably not Debian based) I believe they can return AMD64, rather than x86_64.
I had the same problem as OP on Linux Mint 17.2 64 Bit XFCE. I installed from the sudo and now the game crashes with a report game crash window.