6L6GCs are power amp valves commonly used in American styled amps. They're an octal power valve of approximately 30 Watts of output. The 6L6 is a beam tetrode output valve, meaning they have additional plates to focus the electron stream into beams to increase the output of the valve. They function in a different way to a pentode (like EL34s for instance), giving them an different tonal structure.
6L6s are commonly considered to be scooped and contribute to the thick bass end and crystal clear top end found in many early 6L6 equipped amplifiers of the 50s.