If you install your fonts to the .fonts folder, they will be available for all programs from within your user's session.

For the GIMP specifically, you can also check the Preferences from the menu Edit > Preferences, then Folders | Fonts (see image, below). There you can find out where GIMP's own fonts folder is and add/manage additional font folders.

So, if you want some specific fonts to be available exclusively to GIMP, you would copy the font files to GIMP's font folder.

If you want GIMP to be able to use fonts in your system that are not available system-wide (e.g., installed by TeXLive or other programs in their own folders), you add the corresponding font folder.

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Note: It is not necessary to add the .fonts folder to the GIMP Preferences.