Maybe nappgui is best cross platform gui lib for nim

[Isofruit]

As PMunch stated, looks like a nice project to provide bindings for. You could 
do sth. like owlkettle does and put a declarative GUI-DSL on top of that for 
far better ease of use (this is what makes me e.g. prefer the owlkettle 
approach over gintro: I have an explicit immediately visible widget-tree).