Mix/Glossycoatting bump mapping on GPU now works like CPU: https://github.com/LuxCoreRender/LuxCore/issues/307
As far as I remember, this has been the only limitation and difference between GPU and CPU rendering for a long time.
The general idea is that each sub-node will use its own bump mapping (i.e. glosscoating will use base material bump mapping for the base material and coating material bump mapping for the coated layer).