There are many ways to get a variety of gray/pewter look on Fe alloys and not all alloys will respond in like manner so it's prudent to understand that attachments like sights, breechplug, ect. may show obvious variations. Also, depending on the method, some gray finishes are very fragile in that even light handling will shine them up and some are prone to promoting rusting. I strongly suggest doing a lot of research before deciding on what type and method of finish you desire before doing anything to the gun.
As for removing the rust, an acid like hydrochloric will be the fastest and easiest but not suggested if you're not experienced in working with acids. Using any type of acid will also require complete and thorough neutralization paying particular attention to areas where acid can become trapped. Common retail products like Naval Jelly, Lime Away, ect. are typically acidic and need to be utilized in the same manner and with the same care as a straight acid.
If you're not sure, I'd suggest following the advice someone else gave to just rub-back the brown, if you don't like it, just degrease and let the brown come back.