Impossible but you can reconnect and your mute/gag will be removed. If you are lazy to quit gta and open it agan you can download this cleo in attachments. Press LEFTSHIFT + 0 to reconnect. If it shows any problem after you press LSHIFT and 0 in same time, just try pressing them again untill you enter in server. In time when you press those keys you will be shown as timeout not quit in chat for example Player(id:0) has left server. (Reason: Timeout) and if admin asks you when you rejoin just tell him that you had internet problems.
Credits for cleo goes to Monstercat.