Midgardsormr is right. Just click once on the Foreground color and set your values like the image bellow and you will have "50% grey". You can again make the layer using the foreground color that you know have set or use the fill tool.



Also for the most part most questions you are asking are general software use related issues. We are more then happy to help and answer the questions. But I like someone who tries and teach themselves as well. Here is GIMP's tutorials on the use of the software may wanna refer to it to learn the software as your trying out mapping. I personal referred to it when I wasn't sure on the use of one of the software's tools and more often then not it answered my questions.