Gatestoning probably is the most efficient/cheapest way for 99 magic, but its not free. First you have to train up to 32 magic to be able to use gatestones, you must also have the runecrafting required to make cosmic runes. The magic, which is the hardest part, is done by casting things like confuse until you reach level 32. There is a bot from rsbuddy called AlternativeMagic that will help you reach 32 quickly. Good luck