Since noone has responded here is my view FWIW.
6L6GC has Va and Vg2 ratings of 500V and an anode dissipation rating of 30W.
6L6GB has Va rating of 360V and a Vg2 rating of 270V AND MOST IMPORTANTLY an anode dissipation rating of 19W.
Tubes are quite tolerant of over voltage and in this case I suspect that the voltage ratings are very conservative. About the time that the 6L6GC was released tube manufacturers changed the way that they listed ratings and so the 'GB will almost certainly tolerate higher voltages. The system changed from a "Design Centre" rating system to an "Absolute Maximum" rating system. So the data an the 'GB is likely to be "Design Centre" whereas the data on the 'GC is likely to be "Absolute Maximum"
It is that last anode dissipation rating which would concern me (even if it is a "Design Centre" value) - it would certainly require a lower bias point (lower idle current) so I would not recommend just plugging them in to see what happens.
For some chat on tube rating systems see John Broskie Blog here:
The Tube CAD Journal,New Mu Follower
Hope this is of some value to you.