GG is the friction coefficient rating of a given brake pad. All pads sold in the US should be tested and marked with this two letter code. The first digit is the low temperature rating, second being high temperature rating.
On new pads you'll see the two letter code printed on either the back of the pad or top/bottom of the friction material. I believe (someone correct me if I'm wrong) it runs E through H, H being the highest.
From my own observations, low quality pads (the $15 ones you buy at Autozone) will usually be an "EE" or "FE". When the low temperature rating is higher than the high temp rating, it implies that the brake pad will fade at higher temps. It's better to see an EF than an FE, in my opinion, as you will likely get better performance as the pad heats up, instead of the opposite.
I've seen that ceramics in the $30-$50+ range usually carry a FG or GG rating, which I consider good. Semi-metallics / organics I've seen all over the board-- EE, FE, EF, GF, etc, but I've never seen them GG. I think it largely depends on who made the pad, for what application. You can always ask the clerk at your local autoparts store to see the rating before you buy. If there's no marking, don't buy it! I don't think the code tells the whole story, but you can certainly use it as a guide to see if the pads you're looking at (especially budget ones) are worth buying-- I would be wary of EE, but I've seen pads with great reviews that had an FF.
FWIW, you probably won't see an H rating except on motorcycle pads. I've never seen one in the wild on a pad intended for passenger vehicles.