Discuss the different types of equipment needed for screen printing. Topics include manual screen printing presses, automatic presses, dryers, folding machines, starter kits and high end machines.
I don't know about Gimp but I use Corel Draw and the software separates the colours for me. Try Inkscape (also free) to convert the graphic to Vector. send to print sand there may be an option to print colour separations but I cant't guarantee that.
This specific file is a little too low resolution for anything I would do, but assuming you have a hi-res image, there are a few ways you can go to separate those colors (though I don't know how I would do it using Gimp - love the name).
Using Illustrator
a) Recreate the artwork as vector using Adobe Illustrator or Corel Draw.
b) Change each color to black and print each color out separately.
Using Photoshop
a) Double click the background layer to unlock it.
b) Using the magic wand tool, with "contiguous" unchecked and tolerance about 32 (you may want to play around with this #), click on one of the colors to select it.
c) With color still selected, go to Layer>New>Layer via Cut. This creates a new layer with just that color on it.
d) Double click the layer to get to the Blending Options pop-up. Select Color Overlay, check the box and change the overlay color to black.
e) Repeat b, c & d for each color.
f) Print each color separately.
There are many more ways to separate the colors - some probably better than mine - but this usually works. For this specific artwork, if there is no hi-res image available, I would go the recreate as vector route.
Confused about creating a film positive 
