To be perfectly honest, I think I would've preferred a Neo Geo Pocket Color or WonderSwan Color "demake" of Bertil Hörberg's Gunman Clive to the GameBoy-ified version that can be downloaded from gunmanclive.com, but don't take that as a complaint.
After all, even this pea-green-and-black recreation of the first Gunman Clive's first level is pretty darn awesome.
It was produced by Hörberg himself, by the way. Here's what he had to say about the effort on the site linked to above.
"I've always felt that it's basically a GameBoy game at its core. I emersed myself completely in the world of 8-bit assembler code, four-color tiles and monochromatic screens, and I enjoyed myself tremendously.
"But it ended up taking a lot more time and going a lot slower and than I anticipated. After over a month of intense development I finally have a presentable demo. There's a few more things I'd like to do with it, but I don't know if I can justify spending any more time on it."
I haven't yet tried it for myself, but apparently this ROM can be played using any "decent" GameBoy emulator or even on an actual piece of GameBoy, GameBoy Color or GameBoy Advance hardware--assuming you have access to a working flash cart.