I didn't experience weight gain, but I did experience a weight shift.
On T fat mass will redistribute from the hips and breast will shift to the gut. Are you sure you're gaining fat and that it doesn't just look that way since it's easier to see the gut?
When I started T I thought I was in relativity decent shape, my friends commented on my having a large butt and thighs, but I didn't think much of it. On T I started gaining a gut like crazy. For the first time I started working out, mixing strength and cardio. This helped to get rid of the fat that was always there with the added bonus of increasing muscle mass, particularly in the upper body.