This format can...
read and write waypoints
read and write tracks
read and write routes
KML, the Keyhole Markup Language format, was used by Keyhole and is used by Google Earth.
There are concepts in KML that GPSBabel can't support very well on read because they don't map well into other programs. For example, KML has ideas of camera views and names and descriptions can have arbitrarily complicated HTML in them. KML files may have tiered "Styles" which can identify sizing info and URLs of associated icons. Reading such files with GPSBabel - even if your goal it to write it back out as KML - can often have surprising results. Simple files with waypoints and paths (which GPSBabel represents internally as tracks) work fine.
Google Earth also uses GPSBabel internally for receiver communications and several file format imports and exports.
In general, GPSBabel's KML writer is relatively strong. GPSBabel handles simple KML on read fairly well, but if you're dealing with handcrafted KML that uses extensive features that have no analog in other formats like nested folders, ringgeometry, camera angles, and such, don't expect GPSBabel to do well with them on read.
Google Earth 4.0 and later have a feature that can surprise users of this format. Earth's "time slider" feature controls what timestamped data gets displayed. If you're using data that has timestampes (e.g. GPX points that contain time or almost any track data) this will be important to you. The time slider defaults to the far left position and fully closed. This means that only the first data point will be displayed. You can tweak Earth's settings to "view->show time->never" or you can widen the time slider to show the range of data of interest.
See Google Earth's documentation on timelines for more info.
Export linestrings for tracks and routes.
When this option is nonzero, GPSBabel draws lines between points in
tracks and routes. The default value for this option is 1, which causes
lines to be drawn by default. To disable line-drawing, specify
Export placemarks for tracks and routes.
When this option is nonzero, GPSBabel draws placemarks for tracks and routes.
The default value for this option is 1, which causes placemarks to be drawn.
To disable drawing of placemarks, specify
Width of lines, in pixels.
This option specifies the width of the drawn lines in pixels. The default value is six pixels.
Line color, specified in hex AABBGGRR.
This option specifies the line color as a hexadecimal number in AABBGGRR format, where A is alpha, B is blue, G is green, and R is red.
Altitudes are absolute and not clamped to ground.
When this option is nonzero, altitudes are allowed to float above or below
the ground surface. By default, this option is zero so that altitudes are
clamped to the ground. Specify
floating=1 to allow them to
This option is more useful to pilots than to hikers.
Draw extrusion line from trackpoint to ground.
This option is a boolean flag to specify whether Google Earth should draw lines from trackpoints to the ground. It defaults to '0', which means no extrusion lines are drawn. The option of '1' is, of course, most useful for points that aren't actually on the ground such as those be captured from planes.
Write KML track (default = 0).
This is a boolean flag, defaulting to '0', that controls whether GPSBabel writes the <Track> tag that Google introduced in Earth 5.2 for tracks. You may need to turn this off if you have a KML reader that's confused by new tags or if size is critical.
Routes and tracks without sufficient time data are always drawn as Linestrings and never Tracsks.
Include extended data for trackpoints (default = 1).
This is a boolean flag that controls whether GPSBabel writes extensive data for each trackpoint generated. By default computed speed, timestamps, and so on are written with the default of '1' for this option. If you are writing large tracks and do not value this information, you can reduce the size of the generated file substantially by turning this flag off by setting it to '0'.
Indicate direction of travel in track icons (default = 0).
If set, this options creates directional icons for trackpoints. Arrows will show the direction of travel on drawn tracks and routes.
Units used when writing comments ('s'tatute, 'm'etric,' 'n'autical, 'a'viation).
Units is a simple option. Specify 's' for "statute" (miles, feet, and other things that don't sensibly convert to each other, but are craved by Americans), 'm' for "metric", 'n' for "nautical" or 'a' for "aviation". Default is 's'.
Display labels on track and routepoints (default = 1).
When this option is zero, no labels are added for track and route points. This option defaults to one, so labels are added by default.