My first question would be, what is his ideal weight? What has your vet told you? Is 14 pounds too much?
Since 60% of cats and dogs in the US are overweight, you need to think about how much weight Gus needs to put back on to be healthy. A switch from a high carb diet, to a low carb diet often gets a cat back down to their normal weight. The elimination of all those grain heavy dry foods leads to a better weight.
Average weight for a male cat is somewhere around 11-12 pounds. Average weight for a female cat is somewhere around 10-11 pounds. ECID though. My tiny Delta would be overweight at 8 pounds! A Maine coon cat would be underweight at 12 pounds.
Cats should have a defined waist area, just in front of the hips, you should be able to feel their ribs without too much of a fat layer overlay, there should be good muscle mass around their shoulders, but able to see defined shoulders.
My vet had my civie Monet lose 4.5 pounds. At his ideal weight of 12 pounds, he looks skinny to me. I think a lot of us are used to seeing our cats overweight but see this as normal or what a cat should look like.
Since your cat is diabetic, you want to be careful about putting too much weight back on. Obesity in cats can cause insulin resistance and is thought to be one main cause of diabetes.
There are body condition score charts you can look at to see how your cat stacks up.
http://www.swaynevets.co.uk/downloads/catBodyConditionScoring.pdf
Having said all that, some ways to put some muscle mass back on your cat is to add up to 20% pure protein to Gus's diet. Cook up some chicken and turkey and add that to his food.
Buy some chicken gizzards at the grocery store which will provide some protein but also help his teeth since there is a lot of chewing involved to eat one of those gizzards raw, or lightly cooked. I give my cats half a gizzard, a big chunk so they have to spend some time chewing.
Kitten foods are higher in protein. Fancy Feast has at least two flavors of the canned kitten food.