Most distributions will propose a large choice of fonts in their package manager. The easier is usually to install them this way.
In case you want to manually add third-party fonts (commercial, downloaded…), adding fonts is usually just a matter of moving font files into a directory that is searched by the font system. Have a look at /etc/fonts/fonts.conf (and perhaps /etc/fonts/local.conf) to find out which directories are searched, or look for your operating system documentation. After copying the fonts there, you should run fc-cacheto regenerate the fonts cache.
Some distributions also propose a graphical tool allowing to install fonts from third-party without bothering about the specifics.
Fonts added this way will be available to all applications using the Fontconfig system (such as GIMP).
For GIMP only
You might want to install fonts for use with GIMP only or you might not have permissions to install fonts system-wide. For such cases, GIMP 2.x also looks for fonts in a GIMP specific font search path. The default place where GIMP will look for user fonts is ~/.gimp-2.8/fonts/ but you can change it or add other directories by modifying your gimprc or in Edit -> Preferences -> Folders -> Fonts. Then press the Refresh button in the Fontsdialog and start using your new fonts.
copy archive to C:\Users\XXX\.gimp-2.8\fonts-
Recomend reiniciate gimp program