Most Gerber files include many overlapping entities - traces over flashes, multiple traces used to fill areas and the like. The redundant data does not affect the artwork so little or no effort is made by the PCB tool to "clean up" the Gerber output.

However if you are trying to use this data as input to a field analysis tool you really don't want any more data than absolutely required to describe the metal areas.

GBRUnion automatically unionizes all touching or overlapping data until all you have is a collection of clean contours.

Figure 1 - the Gerber data for the top metal layer of a small IC package

After running this file through GBRUnion the overlaps have been removed by the union operation. Only "clean" contours remain. Data like this is much better for a field simulator input. The user can control the number of segments used to approximate the arcs by two separate paramters: chord error and arc resolution.