Has Kerbal Space Program ever been in a bundle?
I've heard only good thing about it but I am not willing to pay much for an unfinished game.

I do not believe Kerbal Space Program has been part of a bundle so far.

There was mention, and discussion, of the possibility here , a little while back.

It has, however, gone on sale fairly regularly in the past.

You might want to try the demo, if you have not already.

Yeah, give the demo a try. It is a very fun game to play (been playing since February 2012, first version being 0.13.2 which was available for no charge).

For being an unfinished game, you can do quite a lot. Plus, you can expand upon it through mods (unless you prefer stock). Next version of Kerbal Space Program should be out soon that makes Career Mode (only available in the paid versions at present) more playable (or more finished). However, there are still plenty of people who enjoy sandbox. ;)

On -40% steam atm. Wondering whether to get as they mentioned that the game is drm-free on steam. Is it coming to GOG?

fanboy-endorsement incoming...

The game was beyond excellent with loads of content in the early access phase already, and now it even went into beta. Its not a game you that you can really finish. You just keep on pushing your goals further and further. You'll experiment for hours/days trying to get a kerbal onto Mun (my heart was literally racing the first time I maneuvered my intricately designed lander to the surface - the sense of accomplishment you can achieve in KSP borders on the euphoric). After that you might decide to take on the herculean task of building a refueling station in oribit around Mun, or a base on the surface itself . Then there is still the rest of the solar system to explore, which can be far more tricky.
I played for days on end and only unlocked around 50% of the possible rocket objects, gadgets and structures. It was definitely a worthy purchase back then, and it will be a worthy purchase now. I had to stop playing because my laptop couldn't take the strain of the rockets that I was trying to launch anymore, although I've heard that there have been some significant performance improvements since.

I'll say this as someone who absolutely LOVES playing KSP.

It's not for everyone. If you like the idea of real space flight, not the sci-fi approach (gravitational wells are nebulous and Newtonian physics are nonexistent as examples) and want to relive the heydays of NASA, then it's a great game to get. If that doesn't appeal to you then you might not be interested in it. It's not a really steep learning curve (though there is one) but you might get more excitement out of screwing things up than doing it right.

Worst case scenario go to their website and download and try the demo. It gives you a pretty good idea of what you're getting into. It is DRM free even on Steam (you can launch the game directly) so if that's your big decider than you're set.