GIMP is usefull tool but default fonts are not sufficient for your creativity but this is not a limiting factory, you can add any opensource font to GIMP very easily.
Download required font from website. one of the best site to get fonts is http://www.dafont.com
Go to Edit–>Preference–>Folders–>Fonts to identify font folder. You would get two folders
Dont change system default font folder. You can add downloaded fonts to your local fonts folder.
Please note that .gimp-2.8 is a hidden folder so when you open a folder, you need to enable view–>Show Hidden Files.
Once you have copied font files, re-open the GIMP and your fonts should be available.
Please let me know if you run into any issues.