From the top level, SlickEdit supports C# well and we have some support for tagging the Mono libraries if we detect that they are installed on your Linux system.
We don't specifically have support for gtk#. If it is just a C# library, then I'd suggest creating an extension specific C# tag file for (Tool > Tag Files..., Add Tag File...) Then add the source for the library to this tag file.
As far as tool support for your gtk# project, again, no specific support, however, you can go to Project > Properties, Tools tab and configure your build and compile commands. Hopefully, this should get you off the ground.
