I want to make something horror-like, but the only thing that comes to mind is "Melting of skin". So I experimented with GIMP but couldn't find anything. What specifically I want it to look like, is this:
You won't find any "automatic" melting/dripping effects.
You need to learn to make your own by matching 2 or more pictures together, and using layers masks (or whatever it's called in Gimp.)
The trick to show skin as if it was peeled off or removed somehow is to use a picture with bones, and the original picture. You can also use different bones and stick them under the main subject, as long it biologically makes sense.
I don't know Gimp but you can use the same similar basic techniques in Photoshop or Gimp.
When you don't know how to draw something, you don't re-invent the wheel; you simply use 2 pictures and merge them together.
For example these 2 pictures:
Can become like this:
All I did was to put Britney's picture on a layer on top of the skeleton, and then I erased some parts of her face with different brush transparency, using a layer mask. And you can add red to show some tension or whatever fluid you want.
And if you want to add dripping stuff
You can use a bright color and trace roughly some lines and use overlay on that layer. It doesn't need to be perfect.
Then you duplicate that layer and move it slightly up/down and left/right, and add a saturation on it. It's an easy way to get a thick liquid effect with some shine!
You can add some dimension to the ripped skin by adding a line of a darker red as I did on the top of the mouth. That layer is simply in multiply. And then if you want even more realistic effect, you add very light shadow under these areas to give the impression the skin is at a higher level than the bone. And you can also add some lighter color on top of this area to add more light and add more dimension too. I didn't do any shadow or lightning effect in this example below.
You can use embossing and other colors
If you want some alien kind of dripping, you can use other blending modes and add some embossing to the "fluids", and simply change the colors.
You can add muscles or anything other parts and merge them too with layers masks and add shadows
Use layer blending modes, some layers with brush, and lot of layer masks and play with transparency.*
This is what was used.
If you wanted to make an animation with these effects, you'll need to make many of these images with different melting and dripping phases, and assemble them.
* I know the eyeball is not the same color. Her contact lenses fell off.