GUI-Library for microcontroller

I personally used PEG (at work), but it is not for free. You just need to write a small layer of adaptation and use it. You can also look at Qt or minigui.

I would consider rolling your own "immediate mode" GUI. Jari Komppa has a good tutorial about them. It's a lot easier than you may think, and you'll probably find most GUI libraries--even those targeting embedded systems--are a bit heavy-weight for your system.

If you insist on using a third-party library, below are a few I found. I've never used any of them and they are probably fairly expensive.

• emWin
• C/PEG
• easyGUI