Nowadays, many cameras have a setting to avoid the red-eye from the flash reflection, but sometimes they are ineffective. This is one of the simplest tasks you can do using GIMP to solve of one of the most annoying effects that happens sometime when we take a picture.
To correct a photograph for the red-eye effect, carry out the following steps:
Open a photograph with people or animals in it, where one or more subjects have red eyes.
Using any of the selection tools, draw a selection box around the pupil:
Tip
The filter can be applied to a whole layer/image, but it's better to create a selection around the area of the pupil. Be careful! If you have people with red hair or any significant amount of red inside the selection area, then it might be erased. Use the Ellipse Select Tool and enable the Fixed checkbox inside the tool options to select the pupil quickly. Always try to select just the pupils and nothing else.
Go to Filters | Enhance | Red Eye...