Dodge and Burn tool 
might be the tool you are searching for a quick "fix".
I recommend you made a selection of the area to fix to not bleed out.
You need to know that you can pass/drag over and over while moving the mouse with the same click even in the areas already done, but once you've released the click, the next click will affect the area already done.
There are many different ways to do it, which most of them are better, but require more work and knowledge (it's easy, though but it takes time).
Anyway, in case you're interested here is a tutorial, yes it's about Photoshop, but GIMP has those tools as well, but what's more interesting in this video is it will give you some understanding about light and shadows and a better way to correct them.
➤ https://www.youtube.com/watch?v=sRMFYKiXB6U
might be the tool you are searching for a quick "fix".I recommend you made a selection of the area to fix to not bleed out.
You need to know that you can pass/drag over and over while moving the mouse with the same click even in the areas already done, but once you've released the click, the next click will affect the area already done.
There are many different ways to do it, which most of them are better, but require more work and knowledge (it's easy, though but it takes time).
Anyway, in case you're interested here is a tutorial, yes it's about Photoshop, but GIMP has those tools as well, but what's more interesting in this video is it will give you some understanding about light and shadows and a better way to correct them.
➤ https://www.youtube.com/watch?v=sRMFYKiXB6U