This format can...
read and write waypoints
This format supports the on-card format used by the Magellan Explorist 400, Explorist 500, Explorist 600, Explorist 210, and Explorist XL to describe geocaches. Notice what while the format can hold an infinite number of geocaches, the unit will read and silently discard all but 200 geocache POIs at a time.
You should name any file created with this format with a ".gs" extension so the firmware can read it.
This format does support reading .gs files, but it's kind of a pointless operation as the units can't create or edit these files. Round-tripped data is lossy for all but the most trivial cases as many fields are shortened or discarded on write. "Date last found" is discarded on read as there are no logs stored on the device in which to store the date last found.