G Band (G for "green") is a designator sometimes given to Visible Light Bands corresponding to the color green, i.e., whose Wavelengths are in the 510nm region. Sloan Digital Sky Survey data contains information from a G band that appears to be SDSS nomenclature and covers 400nm to 540nm.
The term G band (specifically Fraunhofer's G Band) is also used to designate a (non-green) band covering the range of 430.40nm to 430.61nm, in which Sun light has a number of Absorption Lines which are among the Fraunhofer Lines. Lines in Fraunhofer's G band include Iron and Methylidyne lines, which are prominent in G-Type Star and K-Type Star stars.
The term is used in other ways in communication.