Going wheat free will, in itself, make no difference to your body fat percentage. I can't even begin to fathom why anyone would give that advice! Nor can I work out how 'eating enough' and 'keeping fat off' go together. Quite frankly, gym instructors aren't the best people to take dietary advice from! 
Questions:
Male/female? (I'm guessing female, but your name could be either!)
How old are you?
What is your diet like?
Do you do any weights/resistance exercise?
If you are female, then a body fat percentage of 27.7% is actually considered to fall within a 'normal', healthy range. You need body fat, and women will naturally carry more than men due to their physiology.
If you did want to drop fat, without dropping total weight (which I'm assuming you don't need to with a BMI of 19), then you should look to adding some resistance exercise into your training regime. I think most people will tell you that running alone (without doing 50, 60, 70+ miles per week) isn't great for changing body composition. Weight work will help you to build lean muscle, and increase your basal metabolic rate (BMR). The energy store the 'feeds' the BMR is predominantly fat stores, so you can get leaner at the expense of fat 