The 21-year-old former QPR youngster has scored once in four appearances for the Bees this season and goes straight into the Gills squad to face Oxford on Tuesday evening.
Manager Martin Allen confirmed the signing in a statement on his club's official website and, in his idiosyncratic style, suggested another arrival could be on the way for the League Two leaders.
He said: "This lunchtime we have signed, for one month, Antonio German from Brentford. He goes straight into our squad.
"We are currently in discussions with another player to join us but I am not going to speculate or comment on 'my mate's brother's friend who works down the chip shop said' stories."