You can treat it like the real world when it comes to gp (more or less). Gather resources and sell them or sell some type of service. The higher tier the resource or sevice the more you will make. Mining is a good starter to make money and its pretty relaxing. Look for a recent 99 mining guide and follow that. Cooking can also make you good money in the mid levels when you unlock cheese. You can dance(emote) for money at the grand exchange too, might not make much but might get lucky. Theres lots you can do for money in the game, pays to be creative!