Dual band GP is like two single band antennas have merged, one rotated by 45° arround the z-axis. Connections are the same for the two antennas: radiator is connected to the same hot-end of the connector, and grounds are connected to the same mass.
This way there is no limit to how many bands antenna can be made. In practice, though, most common are two and three band GP antennas.
For measurements, see the “GP” section.