now this was unexpected. jpeg 50 level compression. done here a lot as you can see.
cleaned background
first time here used selective blur to remove some artifacts
healed greenish tones on skin, on face it is escpecially visible
added some small amount of saturation since image is ruined so why not
and cleaned a lot of dirt
if you are look closely i kept artifacts on the eye and highlight on the right leg. you see blur, or g'mic filters, kill a lot of fine details. if you apply them iris becomes muted, and since eyes are somewhat of focus point of anime atworks i decided to left them raw.
hope you like it. enjoy another GIMP edit.
if you have suggestions feel free to post them.

You could mask out that area whilst filtering, but that once again that will increase the amount of time you have to spend on fixing it.

that's what i did.