I bought this game on GOG yesterday, after played the demo on the official site.

I have a problem. Demo version works fine with gamepad (a PS2 pad using a "PSX to USB converter" and Xpadder), but GOG version doesn't work. Why? There is a possible solution?

see http://www.gog.com/forum/unepic/controller_support

I've solved the problem! I've unlocked the game on the official site version using gog serial number and so now I can use my pad.

I've had a similar problem (but with an Xbox-controller instead of a PS2 pad, but still using Xpadder).
Starting Xpadder as Administrator solved it. It's somewhat weird, tough—so far this is the only game I've played that required this.

Sorry, I must be missing something. Where exactly did you register? If you can provide any further assistance, I would greatly appreciate it.

Edit: I have the GOG version, not the demo.

If you go to www.unepicgame.com, you can use the serial number found in your GOG game page to register a copy also there (follow the 'MyZone' link at the top); while not DRM-free, you'll have access to the latest beta version that features some new functions, including integrated gamepad support.
That version can be installed side-by-side to the GOG one, if you choose a different folder, and it can share save game too (you have to copy the save files from one folder to the other)

There's a confirmed patch, on the way to GOG, that should include this and also offline challenges (that were removed at first because they too required server authentication)

Thank you very much for the speedy reply. I think I'll wait for the GOG version. +1