When given a choice between collecting gems or food, choose food. Food gives Domo the gas to go as high as possible. Collect the most food possible and ignore the gems. After eventually getting all the power-ups, you will not need gems anymore. Spend gems on as many foods as possible that give you more gas, for example Veggie Sushi, Gingerbread Cookie, and Cotton Candy. Ignore anything else that can be purchased with gems until you have all the foods unlocked.