Updated Instructions for Joomla 2.5
Thanks to Henning, I've been using this post as a guide, along with others, to do this and there are some differences in version 2.5. I thought I would share them here, as others keep referencing this post as a source.
For references to the codes, nothing has changed in that regard, Henning explains that well above. The path of the files, particularly the admin file, have changed and requires an additional edit to be made to the code of fonts.php.
Files:
Find: libraries/gantry/features -> webfonts.php
Copy this file to your template -> features directory.
Next find: libraries/gantry/admin/forms/fields -> fonts.php
In your template directory admin folder, create a forms folder, then a fields folder inside that and place fonts.php inside the fields folder.
Additional Step Required:
fonts.php requires selectbox.php to run. Since we have moved the fonts.php file into our template directory, we need to change the path reference to selectbox.php so it points to the right file in the Gantry Core.
Open the fonts.php from your template directory and find this code, line 12:require_once(dirname(__FILE__).'/selectbox.php');
Change the code to this:require_once(JPATH_SITE . '/libraries/gantry/admin/forms/fields/selectbox.php');
That should do it, hope that helps.