I think people used their "own" gcd since there was no need for "Big Integers" which are certainly slower, though there was no need for performance either... Furthermore it's good to have written, maybe only once, a gcd calculation.
I hope other Codewarriors will answer.
You have a good solution with "streams" and "Big Integers", which is very well. Go on!

Make sure that your solution handles whitespace tokens correctly. That's the best I can suggest with the given information.