Do you really assume someone can guess the reason for the error from that snippet?
GValue is one of the more difficult part of GTK, luckily we don't need it often. We have some examples for using of GValue for gintro already. When you use it wrong you get messages like above. Please try harder to provide a minimal Nim example and when possible a working C code too, other languages like Python, D, Rust, Go are welcome too. I do consider providing better interfaces for the cases where GValue is necessary currently, but that is not easy, sorry.
