As said, the GC has nothing to do with Guitar Center when appended to the end of a 6L6--it's just a design revision. Most 6L6 types should be compatible with each other, and I think 6L6GCs are the most common ones. I believe 5881 is a similar tube type to 6L6s but they are not straight replacements--most amps using 5881s would require a minor modification to accept 6L6s. I don't know if the same holds true the other way around, but it might.
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