As I understand it, you can upload waypoints, paths, etc. from supported GPS devices into Google Earth. Is there also support for transferring placemarks and paths within GE into a GPS?
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Depends on what type of content you want to download. Many features in GE can be saved as KML, which is just a subset of XML. Other content is served as a network link or other format that is not as easy to save. You can just download and parse through the KML for the coordinates and placemark info. There are also many available tools for doing the conversion to GPX, which is a pretty popular GPS format. GPS Babel can then be used to convert between GPX and the more obscure GPS file formats if necessary. You might find a convertor that goes directly from KML to something your GPS will digest.
If you are outside the US, I can maybe see why you would want to do that. Lack of any good maps.
Google Earth does not have orthophoto adjusted pictures IIRC. Your Lat - Long can be way off in hilly country. Over 100’ in steep country in many cases.
USGS 7.5’ Quad maps are much better. Garmin maps are better.
Many places, it is not satellite photographs you are looking at when zoomed in tight but regular aerial photos. Relief distortion can be really bad.
YMMV
The OP asked about placemarks and paths, so some data may be downloadable in KML format, and use WGS84 projection . Accuracy of these data have nothing to do with the imagery that is used as basemapping. Google Earth allows you to upload precise XY placemark data (and not even consider elevation i.e., “clamped to ground”), but not every KML dataset will be of good quality. Trust me that GE correctly projects coordinates in KML and they will be equally precise when downloaded and viewed in a GIS or on a GPS once converted to the correct file format.
Some of the basemap imagery is orthorectified and some is not. In my area, and in every area I’ve ever checked, Google takes advantage of both private and public data, and tends to keep the basemap* current*, whether that means better imagery depends on your point of view (Haw Haw!). It’s probably true that there is more uncorrected imagery out there, but that is just one aspect (arf!) that people might judge it by, and timeliness is another. Some people just have a different perspective 
(I’ll stop now)
Ok, I downloaded gpsbabel and I’ve been playing around with converting kml files and transferring them to my GPS. However, my GPS can only hold 20 tracks and 50 routes. What I really want to do is take a series of paths in Google Earth (I’ve marked out roads that are not on the map) and convert them into a map to then transfer to the GPS, not just a track made up of waypoints, etc. Any suggestions?
I saw one of those when I was GPS shopping. It’s pretty much perfect, except you pay more for the satellite images. If only Google would make one…
Anyway, I found a topo map set for my shiny new Garmin that does what I need done, so I don’t need to make a custom map anymore.