I've been assuming it means "general poster" (in contrast to OP which means "original poster"). Could also be "grandparent" (in the context of the comment hierarchy of "parent"). In any case, regardless of the specific abbreviation intended, the meaning is pretty clear from context.

GP (noun): the author of a previous comment which is the topic of the current comment. "the GP wasn't sure about the terminology until IIAOPSW came in and cleared it up."