So, if I got it right... libGCM is the lowest level used on PS3... PSGL was derivated from OpenGL ES and call the LibGCM 'methods/functions/calls' to deal with PS3 hardware...

Almost, but not quite. GCM is the lowest level, but it's also available for direct use. So as a developer you don't have to use PSGL, you can use GCM itself and completely bypass the GL layer.

The "OpenGL Everywhere" people can frequently be seen claiming that using OpenGL allows you to target the PS3, but that's not actually true as nobody who wants performance will actually use PSGL - it's just too slow. Instead, developers will use GCM itself.

This is not precisely true. Rage used PSGL to manage states etc but GCM to build command buffers. So while pure PSGL is probably a bad idea, there is some precedent for putting it into production.