To remove glasses glare from photos, you’ll want to use the Gaussian Blur filter. Go to Filters > Blur > Gaussian Blur. Set the radius to about 10px, and then click OK. This will soften the glare in your photo.
There are two ways you can fix glare from glasses using Gimp. The first way is to use the Levels tool to darken the glasses. Create a transparent image using the Gimp Bucket Fill Tool, and then use the Levels tool to darken the glasses.