GR: Guaranteed Reagent
AR: Analytical Reagent
CP: Chemical Reagent
Those reagent 'grade' labels are done by manufacturer and are not standardized; see some examples:
Some companies break these down into very fine scales. Personally, I think about them in roughly 3 grades: 1) ultrapure reagents that are the best of the best, used for the most delicate protocols; these come with a huge price premium, 2) normal academic laboratory grades that are as pure as is efficient - the specific purity depends on the chemical and how separable it is from contaminants, and 3) junk grade for budget experiments in teaching labs. I'm sure other people will bicker with me over this, but it's the system I've always kept in my head when ordering. I'm not certain in all cases that the actual contents you will receive are any different among some of the grades, depending on manufacturing practices it's more about what has been tested.
I don't know if your three grades listed are going to fit into these three categories that I personally came up with, but I'd probably put your GR and AR together in that second/middle grade; CR might belong in the third.
That said, I don't think the purity is going to matter much between those grades for your use; don't spend 3-10X the cost to get something more pure, I'd select something from my "middle" grade. Your samples themselves aren't "pure", and you're not really using isopentane as a reagent in a reaction but instead as a liquid interface for transferring temperature. Higher grades are probably more free of water and other contaminants, but the stuff you pour out is going to get filled with (small amounts of) water from the atmosphere anyways while you chill it. I've never seen a freezing protocol that was explicit about needing to use a very high purity of isopentane.