I think JB originally added Z for zoom level for map tile zoom level, but (having done exactly this for some internal demos) you can totally abuse it for what you want and it works well. Tile38 is a really excellent geospatial database and a lovely codebase.

As for integrating GPU - look at Gorgonia's CU package by Xuanyi Chew that will give you ahead start from using CUDA from Go. Good luck.