You can use this program currently to read a collection of g3x garmin logs and it will output a .kml file with all of the gps tracks overlaid in one place.

You'll need to compile this program with golang like: go build main.go

Then run it like: ./main -out output_filename.kml <log_file_1.csv> <log_file_2.csv> ...

It will produce a .kml file which can be read by google earth and looks like this: