Some guys lose body fat and some guys gain. I suspect that multiple factors are responsible--age, genetics, food intake, and so on. If your appetite increases on T, you can gain fat, but I have a friend whose appetite increased, yet he lost about sixty pounds, or so he says.
I started getting into the exercise habit before starting T, and it was beneficial. Before, during, and after T, I lost about fifty pounds. But I've been so busy the past three months that I have barely exercised at all, and I've regained some of the lost weight. So I would suggest a reasonable and consistent exercise plan, no matter what.