I have a photo made under portrait dish key light. And the model has a little shiny spot on her forehead. Several years ago I used Olympus Studio (if I'm not confused after so many time passed) and one tool easy erased such spots with one-two clicks. I've found some tutorials for Photoshop but non for GIMP. Can anyone point me into the right direction?
One technique often used to deal with those "shiny spots" as well as many other skin blemishes is called frequency separation.
Frequency Separation technique is virtually a process of decomposing of the image data into spatial frequencies, so that we can edit image details in the different frequencies independently. There can be any number of frequencies in each image, and each frequency will contain certain information (based on the size of the details). Typically, we break down the information data in our images into high and low frequencies.
Like in music any audio can be represented in sine waves, we can also break up an image into low and high frequency sine waves. High frequencies in an image will contain information about fine details, such as skin pores, hair, fine lines, skin imperfections (acne, scars, fine lines, etc.).
Low frequencies are the image data that contains information about volume, tone and color transitions. In other words: shadows and light areas, colors and tones. If you look at only the low frequency information of an image, you might be able to recognize the image, but it will not hold any precise detail.
In a nutshell: frequency separation allows you to separate texture from color, particularly the texture and color of a model's skin, and work on each individually before combining them back together.
There are a plethora of online articles that discuss frequency separation and show how to do it with particular applications, particularly Photoshop CS. Many of the concepts can be translated to work with other tools, such as GIMP or other full orbed photo processing applications. Most of these tutorials are fairly involved and beyond the scope of distilling in an answer here. Be prepared to spend some time to learn how to do frequency separation. This isn't one of those "90 seconds to amazing images" photography tips!
I often just use a relatively soft clone brush with some partial transparency. Create some skin texture to go under the highlight, but leave enough of the highlight showing that the lighting still looks natural. It requires the most manual effort and doesn't work super well for overly large highlights, but it works okay in small, limited situations.
It works best if you have sufficient artistic skill to choose source areas that match well and can blend them together well to form a new replacement portion of the image. The basic concept is similar to the frequency separation idea, since you are looking for areas of the image that have similar color and texture to what you need in that area. It does take some practice to avoid reusing the same area too much and to avoid having a "splotchy" look. It also won't work equally well in all cases if there isn't similar enough areas to be used.