I've never heard of a bus girl. A busboy is a job in a restaurant. I worked in a restaurant as a busboy. There were no bus girls. At any rate, the term 'busser' is the usual term in the U.S.
I'm guessing busser is used for both F and M. It sounds neutral to me. I suppose there are bus girls but I've never seen any. At any rate, busgirl, bus girl, busboy, bus boy - it makes no difference how you choose to spell it.
Salesman, sales man, saleslady, sales lady....