I found a similar post to my request on this forum but it's 2 years old and ACR 8.6. I'm running version 9.1.1 and there's no brush functionality under graduation filter. am I missing something or still due to CS6?
There are several new features that are not included in Camera Raw 9.1 running under Photoshop CS6. The last few updates that were applicable to Camera Raw for PS CS6 included support for new cameras but not new features. And I believe brushes in the gradual filter is one of the features that was not made available.