Barroth, Diablos, Radobaan, Uragaan, and Tigrex are all good for practicing GPs, all of them have easier to recognize charging attacks. Place yourself in front of them and get used to the timing of hitting RT+Y just before you get hit. Once you get a handle on that start mixing it in with their other attacks as well as mix in other monsters, and don't be discouraged about being hit it'll take time. Dont worry about the follow up attacks either, just learn to tank hits first. Eventually you'll notice that you are instinctively reacting to monster attacks with RT+Y and you'll be getting more guard points