Skip the weight gainer and use straight protein. Weight gainers have a lot of overall calories and a ton of sugar. If you dont metabolize sugar well (most of us dont) it can easily lead to fat gain. If you're not using this as a post workout shake, try whey powder, peanut butter and/or flax oil, banana, oatmeal, strawberries, blueberries, lite yogurt etc. You can keep the muscle you have and start to minimize the fat so long as you keep exercising as well. A little cardio will not make you too skinny, and if you start to lose too much weight too fast you can always cut back. You really have to find that line for your body on your own since everyone is different, but keeping protein high will help a lot.
Keep your body lean, your blood clean and your mind sharp. -Rollins