Thanks for the question. After 6 weeks your surgery area is totally healed and yes you can do gym and also do other activities. I wish you all the best.
The majority of swelling should be gone in 2-3 weeks. Extreme swelling, especially if it is just on one side, could be a sign of a hematoma. Please alert your surgeon of this so you can be checked out as soon as possible.
The sensory nerves to the nipple are very sensitive especially to stretching caused by the implant which I believe is the most common cause for numbness rather than being cut. Fortunately as the tissues soften, the sensation often returns with in the first year. If your nipples can become erect...