You might not gain any...I didn't. It does change your metabolism and if you are less active with lupus you need to change what and how much you eat to prevent yourself from gaining weight. I am sure others will disagree but after 6 years on prednisone I have gained nothing by watching what and how much I eat. Avoid sweets and high calorie comfort foods. Limit your carbs and portions until you know how pred affects you.
Short term treatment of pred should not be a big problem if you control your eating habits.