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You need to find the folder in which to install the pattern files. To find this folder, open GIMP, open its Preferences, and navigate to "Folders", and then "Patterns". You will see a list of folders where GIMP will search for patterns. Typically, you will find two folders: a system folder and a personal folder. The GIMP documentation says:
By default, the search path includes two folders: a system folder, where items installed along with GIMP are placed, and a personal folder, inside your personal GIMP directory, where items added by you should be placed. The system folder should not be marked as writable, and you should not try to alter its contents. The personal folder must be marked as writable or it is useless, because there is nothing inside it except what you put there.
We recommend installing the patterns in your personal folder.
Ever since GIMP 2.2, you can install .png, .bmp, .gif, or .tiff files as patterns. Not just .pat files.
This repo contains some patterns, but not all of them.
The official repo of Subtle Patterns unfortunately also seems out-of-date: https://github.com/atlemo/SubtlePatterns
There is a script that will scrape the website which you can try: https://www.npmjs.com/package/subtlepatterns