Boxers. They are both strong, and good at what they do. Gymnasts would have a disadvantage because they do not practice fighting.
A professional boxer would kick the crap out of a gymnast, and no offence but alot of guys who go gymnastics are pretty camp so boxers for cert!
Gymnast.. Stronger, more agile, just much more all-round strength rather than just upper-body strength.
boxer.... they know how and where to punch..