The GM_GridGenSetup_t type exposes the following members.
Bounding box to grid to in current proj units. Pass NULL to use the combined bounds of all input layers (the default).
Units to generate grid at (elevation values in other units will be converted as needed)
Flags for grid generation
Grid tightness multiplier (multiplies resolution) for discarding values far from a sample. Default of zero discards nothing.
X resolution (in current proj units) to generate grid at (use 0.0 for auto-spacing determination)
Y resolution (in current proj units) to generate grid at (use 0.0 for auto-spacing determination)