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.