Gen takes a solid majority, if not every fight. In various media he's shown that he's decidedly above Ryu, and fairly close to Akuma in terms of overall ability, even later in his life. That puts him above most conventional Marvel Street Levelers. Though give Shang a few years to train in the Street Fighter universe and watch out.
Gen destroys Shang. I hate to say it, but Gen knows fighting systems Shang Chi has never heard of. And the ones he has heard of, Gen has perfected them a hundred times over. And if it were to go to who is the better chi master, Gen sweeps again. He has increased his durability to survive strikes that Would kill an ordinary human. He can use concentrated energy blast to keep Shang moving in order to tire him out. Gen can destroy cars and boats with his strikes, all with Chi. I think he is the perfect example of a true Kung fu master.