I bought The Inner World because I've heard that there is a Linux version in the making.
Since it took longer than expected, I started to play it by using Wine, but it didn't work that well. I don't blame the developers for that.
Recently they finally released the Linux version inside the Humble Indie Bundle and it's even available for Linux on their website now. Unfortunately GOG still does not provide the Linux version.

Will it ever be available here? And if so, how long is it going to take?

I hope it will take a while.

I've got the official linux client with an humble bundle. It is a .zip file that contains 2 folders, one named 'Linux' the other one called '__MACOSX'.
Within the 'Linux' folder are some .jar files and an subfolder called 'media', but no README. So you should have some knowledge about java to start the game.
I still haven't found a way to start the game fullscreen and some people have issues to access the inventory.

I would'nt recommend this port to other people. Let's hope that GOG releases a better build of this game.

The sequel game has just been released on gog with a GNU/Linux version.
Now I'm not sure if I should get both games on gog or humblebundle.
Is a GNU/Linux version coming to gog.com?

Would like to know this as well. Linux support for all of the Inner World series?

Well, on steam it's already available for linux, but it's already available for over a month there, so....

Edit: according to a post in the steam forum (link below), the linux version is just a port from the android version of the game, but since i don't own that game on steam (or android) i can't verify it or if anything has changed since that post.

(link to the post i've mentioned)
http://steamcommunity.com/app/251430/discussions/0/611701360817206606/#c1488861734112192919