I recently tried to crop a GIF in Gwenview but was unable to because the feature isn’t supported. I’m not very familiar with the GIF format, but I don’t think this would be too difficult to implement.
I’m also not familiar with KDE or desktop development, but I’d be willing to try and tackle this.
There’s also talk about changing the crop tool a bit, but I think this is a different discussion ([Gwenview] Make the crop utility better to use)