The relationship between PARI and PARI/GP is a bit confusing. "GP" is a scripting language for the computer algebra system "gp". The purpose of GP (and gp) is to provide easy access to "PARI", which is a math library in C. So gp and GP don't exist without PARI, but PARI can exist without GP and gp. This is explained here.
Given that GP doesn't exist without PARI, it's almost always referred to as PARI/GP, so pari-gp makes more sense than gp. This also avoids ambiguity between GP versus gp. Right now there is pari, but no pari-gp. There are actually only fourteen questions with pari. These seven are actually about PARI/GP: 1 2 3 4 5 6 7. IMHO these seven posts should be retagged pari-gp. I just read this and I would create pari-gp myself and retag the above posts, but I don't have the necessary 1500 rep. Could someone please retag the seven posts I linked to above, from pari to pari-gp?