Gog games usually feature installers, but when i installed this using GOG Galaxy (2.0.33, by the way), it did not come with a installer, and when i checked the files, it came with a "Uninstaller". I would like to keep the installer because if i ever lose my pc, i can make a physical copy now and in the future, i can install it using it. Help?

Go to your account page: https://www.gog.com/account

Click on The Witcher. Ignore the big blue button and click on "DOWNLOAD OFFLINE BACKUP GAME INSTALLERS" if the links under it are not already displayed. Download all files (see attached screenshot). Voilà, you now have an offline installer.

In the future, when you install the games, you will click on the .exe but all the .bin files need to be in the same folder.

Thanks!

You can download the games installers directly from the website, as ConsulCaesar correctly said, or you can get them from Galaxy.
If you want to use the latter, just follow these steps:
1. Open GOG Galaxy;
2. Select a game in your Library;
3. Click More, next to Install or Play;
4. Choose Backups & Goodies;
5. Select the game under Game Backup Copy.