Hey Twinkissed - for a fast job awesome!
Thought I would mention some techniques - I have Photoshop and don't have/know GIMP, so hopefully this translates ...
To answer the question on how to get a channel into a layer you could ...
-turn off the eyeballs on the other channels then convert to greyscale
-I think select all, then copy, switch to a new layer and paste (I never use copy/paste but I hear some people mention it)
-Use Apply Image to paste a channel into a new layer (GIMP may have something similar)
-Channel mixer adjustment layer with monocrome checked
I would look long and hard at the original to see what the bulges are doing to the shape - his left eye (right side of photo) slants counter clockwise more radically than normal variations in face shapes. It makes me think that it is the bulging paper.
-dup layer so as to have an original to compare with or use peices of if needed
-Levels adjustment layer, set white and black point to bring up to normal tonal range
Then all on duped 'work' layer - this is the best way I have found to get past the mess and still keep the texture and detail of the original photo .... make sure that 'sample all layers' is unchecked!
-Clone tool blend mode set on lighten at 100%, pick a dark line that you know is damage and sample from right next to one end, (if there is a gradation sample from the darker side of the line so that you do not overlighten) and paint over the line. In lighten mode the only thing that will be affected is the dark line. Keep this up until all obvious dark spots are removed. The trick is carefully picking the souce points.
-Clone tool set on darken and do the same thing in reverse to the light spots.
You will end up with faint damage that is blendable/cloneable - whatever your favorite technique is for fixing faint damage - without turning it all to smudge mush ...
Regards,
Roger