how to get a gun if a felony charged was dismissed

If the charge was dismissed as you say you can simply apply for expungement.  If it was part of a larger agreement where other charges were pleaded to, the Commonwealth may argue that to preserve the record it should remain.  However, it is up to the judges discretion at that point.  If your case was simply expunged you are eligible to have the charges removed and can file the appropriate petition with the local clerk of courts.