If you scroll up a bit on that wiki page you'll see GL_RED/GL_R*
and GL_RG*
for 1- and 2-channel images.
GL_RED
: Each element is a single red component. The GL converts it to floating point and assembles it into an RGBA element by attaching 0 for green and blue, and 1 for alpha...
GL_RG
: Each element is a red/green double. The GL converts it to floating point and assembles it into an RGBA element by attaching 0 for blue, and 1 for alpha...
As GuyRT pointed out if you have EXT_texture_swizzle/ARB_texture_swizzle
and/or OpenGL version >= 3.3 you can use swizzle mask to expand out/rearrange your texture color components.