I googled this:
GGA/GSA/RMC and VTG are all NMEA messages, you can read about that here http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=5214 - to summarize it's the data sent by a GPS receiver to the receiver, the fewer the messages the more you can store, RMC is generally enough for picture geocoding, with www.Locr.com for instance, but it you want full navigation information you will need the GGA/GSA/RMC/VTG combo.
The farmer gave permission to place the cache in the field, but the bull charges.