Hello. Galaxy can both install a game and download a backup offline installer. See image below for an example:
https://www.gog.com/upload/forum/2022/09/d0c71918c327f726057097805b927a08a30aea6c.jpg

Oh, I didnt know this, thanks for clarifying, and the parts that are downloaded as backup from Galaxy match the same that we get from site? I have half of a huge game so I want to know if I can get the missing parts and get it fully that way

Yes, the installers are the same and Galaxy can resume interrupted downloads, as long as the download was initiated with Galaxy itself. I'm not sure, however, if it can resume downloading from browsers. Let me give it a shot.

Edit: OK, I just gave it a shot. Galaxy cannot resume incomplete parts from the browser, but if some parts of the installer were completed, they can be placed in Galaxy's download directory (it is a bit rough of a procedure, to be honest), so that you don't have to download them again.

Thank you for explaining, and oh, that sounds really good then, even if it cant resume browser download if the ionstaller parts are the same thats what I need as I did download some parts after a long time retrying them etc And browser download never manage to be resumed so if Galaxy canresume its own downloads then its good for me

GOG and GOGGalaxy builds are differ although suppsoed to be same (GOG->GOGGalaxy to match and been compatible to add later). Somtimes Galaxy versions superior, sometimes by mistake - offline versions.

I have a certain problem, I tried Galaxy from the page of the game, download it and then there is no option to download the backup files, it starts getting the whole game, in fact it says it will install meaning that will download to temp folder and then delete i guess, am i missing something? the game is Cyberpunk 2077 just in case

Except for Gwent, all games have offline backup installers. To access an offline installer, go to the game's page and click the button called "Extras" which is to the left of the purple "Install" button. Then you should see the page for the offline installers. Let me point you to the right direction using Witcher 3 as an example:
https://www.gog.com/upload/forum/2022/09/15ca12995573fb945b62e4456e0b733234af4ac3.jpg

If the game has a lot of goodies (like Witcher 3 does), you may need to scroll farther down to find the offline installer.

I found the section you mentioned, thanks again, the issue is this game in Galaxy has a huge single file download instead of the 28 parts or so that makes the offline backup from site. My issue is right now, aside from having to download half of the game again, also I dont even have 108gb for it, I guess thats like it has been said before that Galaxy and site backup version may be different

Like I said above, the offline installers downloaded through Galaxy are the same as the offline installers through the browser. You are not downloading a single file through Galaxy, you are downloading all the installer's files in a single download.

Have you tried DownThemAll as your download manager add on, and see if that works better than the ones you have already tried ?.

Ah yes but its not allowing me to pick some files from the single download, I mean Im sure you are right and in the end I get the same files just it gets them all at once I guess

I will try DownThemAll now, maybe can work, who knows, I noticed that the free download manager while its unable to resume as is, can resume if I decide to download a file again and it has a partial download of it, so its not very automatic process but still could save some time using partial downloads, but as said will try your suggestion

EDIT tried it and it renames the files in an odd way, I already will have it rough considering I have some downloaded and some not, then maybe I will stick with free download manager as I can resume manually if I decide to download something that was halted before, still will take some time as it will not automatically resume or pick second file after first is done because links seem to be different like every few minutes, in fact once I got a different build version oddly

If you are on low of space - both game and system drive's - yo better use GOG Galaxy. 108 Gb come with all languages and require same amount of space as it sized for TEMP locations on system drive.

Thank you, I will keep that in mind, certainly seems useful to know this