Minimal requirement is support for Linux.
The most important is that it is usable in production and open-source with decent documentation (so amount of bugs and problems will be low and once encountered fixing it will be possible).
I would prefer a cross-platform GUI library, with native look and feel.
It is necessary to support Linux, it is preferable to support also Windows and Mac. Supporting additional platform would be nice.
It is also strongly preferable that it would easy in use and do not require huge amount of boilerplate code.
Library in active development would be better.
question based on https://stackoverflow.com/questions/260905/whats-the-best-easiest-gui-library-for-ruby (it was closed there as offtopic, there are some answers).