Unfortunately, changelogs are, officially, only accessible through one's account if they own a game.
But as think that's not in the right direction, the community tries to make them publicly available.
As mentioned already, you could check the "What did just update"
thread, though finding the latest changelog for a specific game would require some time and effort.
We also try to post them in the game specific forum - you can check for sticky threads called "Changelog".
There is a Community Wishlist
entry for them to be posted in the game specific fora, if you want to vote for it.
As things are, your best bet is to visit
(this is a feature-rich search engine [url=https://www.gog.com/forum/general/magog_a_search_engine_for_gog_games/post1]mrkgnao created and maintains), search for a game you're interested in, and check out its changelogs - it's the link right above "Minimum System Requirements".
For example, if you search for
Baldur's Gate: Enhanced Edition you'll get
this page.
Hope this helps.