Can Gnar catch enemy Gnar's Boulder/Boomerang?

Sorry, but no. They damage each other, not allow the other to catch it. Gnar would be a huge counter to Gnar if that were the case.

Based on what I've seen and how the boomerang operates, I doubt that is/would be a function considering it would mean that an enemy Gnar could throw his own, then tank a boomerang hit from you and catch it with extreme ease and be able to throw another one before his even returns with the 60%cdr it gives when you catch it (specially if he opts for cdr runes/masteries/items)