Disclaimer: I've never used GIMP, so sorry in advance if the things I say don't apply here
Well... It's not a masterpiece, but it looks pretty decent for a custom board for playing with friends.
A couple of suggestions for next time:
Don't ever ever use the pencil tool! In the original picture it's very clear it has no softness to it which makes it look pixilated. Unless you are making an early 90's Windows game, you don't want that.
Only use brush tool and spray tool. Be sure to set the brush to have a certain softness so you don't get those jagged pixels.
I generally find it a nice thing to make the picture in double size. Makes it easier to paint certain things and go into detail.
Then when I'm done I scale it down 50%. This makes different parts blend together very nice.
When making cracks in the stones, make a thin bright line along one side of the cracks and a dark shading on the other side.
Notice how the light direction is consistent throughout the tile. It's important to add depth to the details. If not they are just lines on the tile.
You could also consider making a new layer and use the flood fill tool with a gradient to create a sense of illumination. It really makes the room come more alive.
I have exaggerated a bit here to show what I mean. It should be more subtle in my opinion. Of course it all depends on the look you want to achieve