and I was wondering if someone could tell me how to make backup game data saves for my GOG games

1. Go to https://www.gog.com/account
2. Click on the game
2a.: If the heading says GOG Galaxy installers: Click on More, then Classic game installers
3. Download all the files and back them up somewhere safe

OR

1. Start Galaxy
2. Go to library
3. Click on the game
4. Click More, then Backup and Goodies
5. Click on the files.

Is this about making backups of the saves of your installed games (using Galaxy or not?), or backups of standalone game installers?

Save games are just files (or folders) in a given save location. This can be in several places, for instance (and I assume windows here):
MyDocuments
Game install location
Type in a file explorer: %appdata%
The above one is abnormally hidden application data storage area.
This should cover most savegame locations but google if you can't find it and this site is very hseful:
https://pcgamingwiki.com/wiki/Mount_%26_Blade
I have use m&b as an example, but it tells you where exactly to find things.

If it's about the save games, all three Witcher games store them in your Documents folder:

/The Witcher
/Witcher 2 (note that a The Witcher 2 folder might also exist, but it's empty, at least on my machine)
/The Witcher 3

Unfortunately with all the different publishers here and with the different OSes supported, it's hard to come up with a specific solution for you.

What is your specific OS that you are running?

As to a list of where publishers put their save files, folks here have tried a couple of times previously to come up with a list but we usually just wind up pointing folks here:

http://pcgamingwiki.com/wiki/Home

Example of one of the previous threads:

https://static.gog.com/forum/general/gog_really_doesnt_make_it_easy_to_backup_saves_and_achievements