GOG recently said one of my games got an update, i dont use gog galaxy and was wondering how updating work. Also i heard that games on GOG don't always get updates from devs and i was wondering if someone could explain that to me.

Unfortunately, that point is quite true.

Many devs abandon their games on GOG.

When this happens, GOG usually to always does absolutely nothing about it. And so those games usually remain abandoned & un-updated & broken on GOG forever.

GOG even makes new deals with companies that abandon their games on GOG, like for example, the recent deals GOG made with Capcom to re-release the original Resident Evil games, despite the fact that Capcom has abandoned the GOG version of Dragon's Dogma 1 for eons, for which the GOG version is permanently missing vital game-fixing patches that the Steam version has.

How often do games just get abandoned on gog? I was wondering becuase so fare i have rather enjoyed GOG and i would like to avoid buying abandoned versions of games.

Not all games that are "abandoned" are abandoned permanently. Some devs and publishers are just slower to update stuff here than on Steam or other exclusively launcher-based stores/services. There's a community thread (focused on a user-maintained spreadsheet) that aims to catalog the various ways in which individual titles sold here are currently behind, or otherwise worse off than, those titles on other PC services (but mostly Steam).

EDIT: Fixed link.

Usually you have to re-download the installer from your library and reinstall the game in question. Sometimes in rare cases GoG offers delta patches in the download section below the installer files.

Go to your library (the same place you downloaded you installer(s) from) and open the "card" for the game in question. If it has (a) separate patch(es), download the appropriate one(s) and run it/them in sequence from older to newer. (Mind the version numbers, though -- if your installed version is older than the one(s) the available patch(es) was/were designed to update, you'll have to just download the whole installer.)
If there's no patch(es) -- or your installed version's too out-of-date to work with the available patch(es) -- just download the full installer, which will always be the most up-to-date of the "offline backup" stuff. There's usually no need to uninstall the old version before running the new one, either -- it shouldn't overwrite anything like user saves or settings.
Just know that sometimes it might take a little while -- say, a day or two -- for a game's offline installer to get updated after the latest version is already in Galaxy (since devs can push Galaxy updates to users directly, but GOG staff have to process and test the offline installers, which is not a completely automated process). It's pretty rare for the installers to be left out of date for a long period, though it happens from time to time.

Do be aware that merely opening a game tab in your library will clear any "new" flag or "update dot", regardless of whether you download anything, add tags, etc. So if you ever want to get into the habit of backing up games shortly after you buy them (not a bad habit to get into), you might consider making a custom tag that you can use to mark games that you haven't downloaded at all, or at haven't downloaded since before a newer version became available here. (Unless you wanna be super serious about it and start a spreadsheet or something.)

EDIT: Damn, ninjaed.
Edited for coherence and clarity.

Since nobody actually explained it: It depends on the game.

Some developers are nice, and even go as far as to offer a patch of the Linux version, which is very thoughtful of them.

Others, you'll have to reinstall the whole thang ding, even if only one or two files (overall) changed.

You are trolling.

Thank you i will just have to check that spreadsheet when i buy a game. So fare only one GOG game i own is on that list and that is due to a missing linux build.