The Gazillion option enables support for loading, viewing, and simulating patterns from GDS-II layouts.
In HyperLith, a GDS Multi-site pattern allows you to load a GDS-II file, select a structure and layer to view, and then choose one or more regions from the layout to simulate.
EM-Suite supports loading multiple GDS files, selecting and viewing multiple layers, simulating multiple layers, and overlaying simulation results on the source pattern.