GIMP Brush, or GIMP Basic Brush, is one of the formats used by the GIMP image editor to store a brush or texture. It is effectively a raster image format.
There is a third kind of GIMP brush file, GIMP Parametric Brush (.vbr), but it is probably of little interest to applications other than GIMP.
The usual filename extension, at least for current versions of GIMP, is .gbr. Some sources also list .gpb.
The ASCII characters "
GIMP" appear at offset 20.