I guess u have to export it from zbrush as OBJ-File to an other modeling tool and then convert it into an 3dgs-compatible format.

The main advantage of zbrush is the high poly-count u can work with.
But in games the models have to be low-poly.
You can use your highpoly models from zbrush to calculate normalmaps which u can use in 3dgs with a shader.

cheers

sinnlos