In Galaxy go to 'More >> Configure' and select the DLCs.
if u mean the menu in gog that shows my achievements and play time in witcher 3 then there is not such thing as "configure" in "more". there are settings. but it only allows me to change between auto and manual updates
if u mean "backup & goodies" then the only position there other than artbooks etc is GOTY. no option to download dlcs
I said Galaxy, and I meant Galaxy. The client is not call "gog", it is called Galaxy. Having said that, I don't use it and I was going on what one of the support articles says, namely:
" If you're using GOG Galaxy:
--Select The Witcher 3: Wild Hunt from the sidebar to open its game page.
--Click on the drop-down labeled "More", (located next to the PLAY button) and choose "Configure".
--The Configure page will feature all available DLCs listed towards the bottom.
--Select each DLC that you want to install, and GOG Galaxy will do the rest. If you want to remove the DLC from your game, simply uncheck the selection box."