All gliptins reduce the spike you get from your food.
I suspect your insulin resistance is high and because of that your body is storing it as excess body fats, which means that you have increased weight over time.
What the inhibitors do is take the sting out of the spike and that reduces the need for more insulin to be produced. Less insulin less weight gain.
What I would suggest is that you reduce your carbs slowly until your body adapts to low carbing, reduce your plate size, as portion control is important. And do a little bit more exercise however you can with your arthritis!
Even something easy lying down or sitting!
It will all help!