The midfielder scored a superb volley to open the scoring at the City Ground before Anthony Elanga equalised with his fifth Premier League goal of the season.
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The visitors dominated possession and had their fair share of chances before retaking the lead through Fabian Schar.
It appeared Newcastle would hold a 2-1 lead at half-time but Callum Hudson-Odoi pounced in the sixth minute of stoppage time to level the contest.
Forest then thought they had a chance to take the lead as Taiwo Awoniyi went down in the box, but referee Anthony Taylor overturned his decision to award a penalty after consulting the VAR monitor.
Minutes later Toon went up the other end as Guimaraes brilliantly intercepted a loose pass before bending a winner beyond goalkeeper Matz Sels.