gimp/devel-docs/ggr.txt

    The .ggr file format.

GGR files are used to store GIMP gradients.  A gradient consists of a
series of consecutive *segments* spanning the range from 0 to 1.  Each
segment has the following attributes:

Left Endpoint coordinate: (double)
Left Endpoint color (RGBA color)
Right Endpoint coordinate (double)
Right Endpoint color (RGBA color)
Midpoint coordinate (double)
Blend function (enum; values are 0 = "linear", 1 = "curved",
                2 = "sinusoidal", 3 = "spherical (increasing)", 
                4 = "spherical (decreasing)")
Color type (enum; values are 0 = "RGB", 1 = "HSV CCW", 2 = "HSV CW")


A GGR file is an ASCII file structured as follows:

Line 1: "GIMP Gradient"
Line 2: "Name: " followed by the name of the gradient
Line 3: the number of segments 

The remaining lines consist of segment specifications.  There must be
one line for each segment.  Each line contains 13 numbers -- the first
11 are floats, the remaining 2 are ints.  Here is what each field
encodes:

Field      Meaning
  0          Left endpoint coordinate
  1          Midpoint coordinate 
  2          Right endpoint coordinate
  3          Left endpoint R
  4          Left endpoint G
  5          Left endpoint B
  6          Left endpoint A
  7          Right endpoint R
  8          Right endpoint G
  9          Right endpoint B
 10          Right endpoint A
 11          Blend function type
 12          Color type

The left endpoint coordinate of each segment must equal the right
endpoint coordinate of the preceding segment.

Note 1: This is a description of the *new* gradient file format.  In
earlier versions of GIMP a different format was used.

Note 2: A few of the gradients at the top of the list in the Gradients
Dialog are special ones, for which shades of gray are replaced with
proportionate blends of the current foreground and background colors.
These special gradients are not represented by .ggr files in the
"gradients" directory, and there is currently no way to get this
behavior for a custom gradient.

Note 3: Starting with version 2.1, Gimp now has the ability to load
gradients in SVG gradient format, if the file is placed in the user's
personal "gradients" directory, or some other location in the
gradients search path.