> Is "GP" a variation of "OP"?

No, GP is “grandparent”, that is, the post two posts upthread from the post in which the term is used (or, in some contexts, the user who posted the post two posts upthread.)

Similarly, GGP is “great grandparent” (3 posts up), and so on.

OP is “original post” or “original poster”, which is (in the general case) a different thing than GP.

Thank you, I've been curious about this for a long time. Is this a new thing or did I miss the boat on some old message board thing?

I think I first encountered it on Slashdot a decade or more ago, but I've rarely seen it outside of Slashdot or, more recently, HN, so I think it's a Slashdottism that migrated to HN with Slashdot users.

Though I could be wrong, as my exposure to internet message boards is weirdly eclectic, but far from comprehensive.