Yes, you are absolutely allowed to carry a Garmin GPS device on a flight, but there are specific rules regarding its use. As a "portable electronic device" (PED), you can keep it in either your carry-on or checked luggage. If you carry it in your cabin bag, most airlines and security agencies (like the TSA) permit it because it uses a "passive" receiver to listen to satellite signals. However, during takeoff and landing, you may be asked to stow it. The biggest catch is that many airlines prohibit the use of GPS devices during the flight because they can occasionally interfere with the aircraft's sensitive navigation systems, although this is rare with modern shielded electronics. Additionally, a GPS often struggles to get a signal inside the thick metal fuselage of a plane unless you are sitting directly next to a window. If your Garmin uses a Lithium-ion battery, it must be kept in your carry-on luggage rather than checked, as spare or high-capacity lithium batteries are a fire hazard in the cargo hold.