GIMP is just like any other image manipulation program like Photoshop, Coral Painter and Paint Tool SAI; the only difference is this program is free.
Anyone can use GIMP no matter what the skill level. It is more complex than MS Paint by far but more simplistic than other programs making it easier to navigate and get use to.
Things you can do with GIMP include photo manipulations, digital painting, inking, texture creating, text creating, and animating. There are a plethora of other tools including many types of blurring, an iWarp tool that works like the liquify tool on Photoshop and layer modes that make it easier to create your art.
A mouse or tablet can be used in conjunction with the program and a mouse can be used in all cases- tablets aren't needed and can be circumvented for painting with the opacity setting on tools and layers.
Brushes may be a concern for some but not to worry! New brushes are made all the time and can easily be added to the list of current brush