The GM_WatershedParams_t type exposes the following members.
Filename to save filled DEM to as a GMG file (typically NULL or empty string)
Flags with misc. options
Maximum depth of depression to fill in meters (0.0 for no depression filling)
Drainage threshold for how many cells have to flow to a cell before it is considered part of a stream.
Grid cell spacing in X dimension in specified projection coordinates (use 0.0 for default)
Grid cell spacing in Y dimension in specified projection coordinates (use 0.0 for default)