There is also .otf type fonts.
Definition: Developed jointly by Adobe and Microsoft, OpenType fonts technology is an extension of the TrueType font format but also can contain PostScript data. OpenType fonts are cross-platform, the same font file works under both Macintosh and Windows Operating systems. This digital type format offers extended character sets and more advanced typographic controls. Like TrueType, a single file contains all the outline, metric, and bitmap data for an OpenType font. Although any program that supports TrueType fonts can use OpenType fonts, not all programs can access the full features of the OpenType font format at this time.
OpenType fonts install in the same manner as TrueType fonts and co-exist peacefully with TrueType and PostScript Type 1 fonts.
My question to you is: Where are you placing the font file? They should go under the C:\WINDOWS\Fonts directory in order for Windows to see them. However, GIMP may have a different location that it looks for fonts. I know when I use Adobe Photoshop, the default location is C:\Program Files\Common Files\Adobe\Fonts.