It is not gray or black. It is the original game. It is legal to download, even if you live in Spain. If you just want to play the original game on your own local network, this is how you do it.
Abandonware isn't technically legal, it's simply that the rights holders do not prosecute (or do not exist). If someone has bought the rights to the original M.U.L.E they would be in their rights to sue for copyright infringement. http://en.wikipedia.org/wiki/Abandonware
Does anyone know whether the original M.U.L.E was released into the public domain?