The 18-year-old Forest academy graduate signed his first professional deal with his parent club last season.
He impressed on loan with National League Wrexham earlier this season, scoring five goals in 21 appearances.
"Gerry brings an added physicality to our forward line," Cambridge United manager Shaun Derry said.
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Nottingham Forest striker Gerry McDonagh has agreed to join League Two side Cambridge United on loan until the end of the season.