Newton the Gnu, codenamed Agent Silent G, is an O.W.C.A. agent first appearing in "Agent Doof". He later appeared in Phineas and Ferb Save Summer, and fought the scientists with the other agents. During both fights, he suffered, because one scientist punched him and threw him down, and another made him crash into a wall.
He is titled Agent Silent G for the silent g in "gnu". Coincidentally, Agent Silent G doesn't seem to make a sound.
In Brazil, the g in "gnu" is pronounced so Newton had his name changed to Gnewton.
Newton's identity remained secret until Doofenshmirtz put it up on his blog.
According to his secret file, he tends to keep to himself. He is known for his full luxurious beard. And he is looking for a new secret identity since it was revealed by Dr. Doofenshirtz.(Where's My Perry?)