Hi. I just got physical copy of GOTY edition, it says version 1.31A (22 march 2017). But every time i start game it tries to download a 34GB update. I checked the game size on my HDD and its 35.9GB. I can play the game just fine, why does it try to update whenever i start the game? I have to abort the download every time. Any help would be appreciated

I don't know why that happens but in Galaxy you could disable auto-updates for TW3 if you don't want the patch to go through. To do this open Galaxy, click on TW3, then on the “MORE” button next to “PLAY”, Settings, and then under “OVERRIDE FEATURES” uncheck “Automatically update to newest version”. If you want to you can also disable all auto-updates if you click on the white gog.com icon at the top left. The setting is under "Settings/Features".

I tried to do wat teleguy said, but it didn't work, the game wont load if i move the files into "program files (x86)/GalaxyClient/Games". Also i've been reading people having much larger install size (50+ GB), whereas i only have 35.9GB. Am i missing dlcs? This is my dlc folder, does it look right? Am i missing texture packages? The game looks high texture though.

My DLC folder looks the same and the whole game folder is 38,8 GB. However I have it installed since release so there could be some junk files left from all the patches since then.

My link doesn't say anything about moving files to "program files (x86)/GalaxyClient/Games".

My Witcher 3 installation with all DLCs is 39,4 GB in size. This includes a hires graphics mod though.

So it that a 3rd party mod? Where do i move the files to in the galaxy client folder?

I have an unaltered installation of the GOTY version and it's 35.9 GB in size.

You can have the GOG client check the files to see if anything is wrong with the data. Or import the installation folder if it hasn't registered them yet.
Open the client and click on the game on the left. Now you should see a screen with a big "PLAY" button and a button that says "MORE". Here, you can manage the installation, i.e. import your folder, check your files and other stuff. Try and see if you can make it work.