Well first of all i managed to solve my problem before posting. But still i want to share my problem with galaxy and a specially with witcher 3 DLC. First DLC maned yennefer alternative look or sth similar is downloaded and then i can activate it in-game option menu. I was looking for a long time why i can not see any part of the wols school gear part... and after "investigation" i found out that u have to activate this in galaxy game configuration - and almost all of DLC i downloaded earlier was not active. WHY one DLC is enabled in game and others on config outside of game. Why its not gathered inb one spot and show clealy whitch one is active and whitch is not. How the hell should i know that i have to atvivate all downloaded DLCs??
cheers.

Not all - just some

Imho gog galaxy is an alpha version at best. After it killed my witcher 3 installation a few times by randomly downloading patches it had already installed a few hours ago i downloaded and installed the patches manually (from account -> library) and never had any problem since. DLC don't need to be activated this way also (except those in the ingame menu of course).

Galaxy is a POC and shouldn't be used, hell i wouldn't even call it alpha

Works fine here. No problems tbh. From Galaxy Setup, first attempt at preload (which I stopped once I knew where it was going) reset, re-preload. Not a problem. Not even a problem with Config either. First three were already "checked" for download and install, which is what that does fine here.

Not sure what you mean because every time new DLC comes out I've installed it through Galaxy by clicking on it in the Configure tab and when I go in the game, the DLC is active immediately without any need to do anything in-game to access it. Whether you download the DLC files and install them locally yourself, or you install them with Galaxy by clicking on the checkboxes for them in More->Configure - either way the DLC gets installed the same way and is enabled by default. There is a DLC option menu in the game but you should find that all of the DLC that puts an entry in there is enabled by default upon install.

If you're getting some other result, I don't know what to say. Every DLC has worked from day one hassle free for me, I just install it, and it just works with zero configuration required.

Update:

I just re-read your post and I could be wrong but from what you've said it sounds like you have been downloading the DLC backup files through "Get Backup and Extras" which does nothing more than download the files to your hard disk and leave them sit there - it does not install anything. Then you're going into the game and not finding the DLC and wondering why perhaps? If this is the case, note that "Get Backup and Extras" does not ever install anything, it is just a straight file download. If you download anything this way, you have to go and run the executable files yourself afterward to install the game/addon/DLC/etc.

In short, there are two ways to use GOG. 1) Download your games/DLC manually either through the website, GOG Downloader or Galaxy, and then manually run the executable files once they're downloaded. Which of the 3 download methods you use makes no difference because you get the exact same files either way and running the installers is the same - no differences. Option 2 is using Galaxy to install everything, in which case you click the "Install" icon to install the game from your library, which does not use any files you may have already downloaded but instead downloads the game direct from GOG's servers in pre-installed format and verifies their integrity. To install DLC using Galaxy for a game you go to the poorly named "More->Configure" page and any DLC you own for the game should show up there. They should all have empty checkboxes beside them, and checking a box immediately causes Galaxy to connect to GOG to download that DLC and install it into your installed game (whether or not you may have downloaded it manually via the backup option #1 above, which does not install anything).

There's also an FAQ for this in the forums. I think they might not actually need an FAQ though if they improved the user interface to just add a DLC menu item because that is what a normal person's brain is looking for when they want to install DLC.

Either way though, while it might not be clear at first, it is not difficult either, and no configuration is required - just installation via one of the two methods.

Hope this helps.

hyh yea i did it through "Get Backup and Extras" ... coz it is extras :")) I thought i was the right way to install extras - i did not see the dlc in other places. The main problem was when i installed firts 4 (i can be wrong with number) DLC they were downloaded installed and activated but all next ones were just dowloaded and i had to activate them manualy through configuration. My main point is that all of thiss is much confusing and there must be easier and simpler way to make DLC menu. Any one more point - when i install official DLC as a custer i want just to click and make it run - not reading faq's , tons of forum or sth.
Cheers :")

Get Backup & Extras has never auto-installed anything, it has always been just download. You have always had to mandatorily run the executables yourself to install anything downloaded that way.

I'm not sure what happens if you own DLC and do a brand new fresh install of a game with Galaxy. On Steam if you install a game that has DLC which you own, Steam will automatically install all of the DLC too and you can uninstall it later if you don't want it installed. I haven't tried this with Galaxy yet because I installed The Witcher 3 the day it came out before there was any DLC. I do know that every new DLC that does come out is not automatically installed though (nor would I expect it to be as that would be a horrible default, especially when savegames made with DLC installed then permanently require the DLC).

I've been using Galaxy since early alpha though, and the only way DLC has ever been installed is by one of the two methods I mentioned previously - download files and manually install, or by checking the checkboxes on the configure page and waiting for it to install the DLC. The only case I haven't tested is if you install it for the first time and own the DLC already, whether it will install it automatically like Steam does, or whether you still have to manually click the checkboxes. Perhaps someone else could comment about that.

Oh, I totally agree with you as per reading FAQs, forum posts etc. IMHO, if any significant portion of users of any skill level need to go read an FAQ to do something as simple as install a DLC addon to their game, then the user interface is not intuitive enough. The proper solution IMHO is to make the user interface so simple that it is practically impossible for someone to NOT be able to figure it out. If a manual explaining how to do something like this is needed, then the software fails, double so if the user is left to hit google to figure it out.