
Gimp - Easy and Simple Abstract Background
Part 1: (The Circles)1) Make sure your Foreground, and Background colors are black and white.
2) Select your gradient tool, and change the shape to
Radial and the repeat to
Triangular Wave.
3) Now give that layer a small gradient.

Gimp - Easy and Simple Abstract Background
Part 2: (The Effect)4) Duplicate the layer, and add another gradient to that.
5) Change it's layer mode to
Difference.

Gimp - Easy and Simple Abstract Background
Part 3: (The Coloring)6) Create a new layer, and fill it with whatever color you'd like.
7) Now change it's layer mode to
Overlay.

Gimp - Easy and Simple Abstract Background
Final Result:

Gimp - Easy and Simple Abstract Background