Before anything else, let's take a look at GIMP and its interface. It's important to know a few basics about it, even if the focus here is not to teach how to use all features of GIMP, but just the main tools and tasks related to post-production work.
The interface is made out of different windows; each window has holds, tools, and options to edit or organize the image. Here is a list of the main windows in GIMP:
Tool option dialog: This menu holds the main options for GIMP, such as the File menu, and the main tools such as selection and painting tools.
Image Window: Here, we have the canvas of the image, which has the actual composition. When we do any kind of adjustment, it's here that we will see the results. Most of the color controls are placed in this window tool.
Layers dialog: As the name suggests, here, we find options related to layers.