"Studying is uninteresting."
공부하는 재미없습니다 is missing a subject. 공부하는 것이 재미없습니다 and 공부하는 것은 재미없습니다 would be correct, I think.
Note I replaced 걸 with 것 and added object or topic particle.
I don't know which sounds more natural. Probably depends on the context.
"I don't love studying. I hate studying. I like learning. Learning is beautiful." - Natalie Portman
the 어요 ending is the polite version of the 습니다 formal ending. the two sentances are the same, just used in different company. basically strangers vs acquaintances... but its more in depth than that.
없어요 is a middle formal style and 없습니다 is an official formal style. So it's both corect.