Why do I see several GIMP windows?
When you launch GIMP, you have multi-window mode instead of one.
The main central window contains the image you are working on. The other windows typically are the Toolbar (on the left) and the Dockable dialogs (on the right).
These windows are just separate panels. You can manipulate them in several ways: turn panels off or move them easily to compose any panel and even make a bond of panels.
In order to compose the panel with the toolbar just click on its title and drag it to the area of navigation panel or to the tab area.