MAIN QUEST SPOILER ALERT! After completing "The waters of life", in the main quest, you can earn the training for Paladin Gunny. He will be in the courtyard. When you ask him, he will ask for Lyon's permission. Go and find elder Lyons, and ask, he will state that you can. Go back to Gunny and ask again. He will train you. END OF SPOILER ALERT! The only other way, is to complete the Operation Anchorage Simulation, in the Operation Anchorage DLC.