Gravelle was born in the Welsh village of Trimsaran in Carmarthenshire. He attended Ysgol Gyfun Y Strade. He was a member of the Llanelli Youth Theatre between 1997 and 1999 before becoming a student at the Royal Academy of Dramatic Art.
In 2011 he appeared opposite Rhys Ifans and Vanessa Redgrave in the feature film Anonymous, which contests the authorship of Shakespeare's plays, as Christopher Marlowe. Gravelle missed attending the film's London premiere due to starring in a London production of Mike Bartlett's National Theatre production 13.
In 2012 he joined the main cast of the ITV period drama series Mr Selfridge as Victor Colleano, assistant manager of the department store's restaurant. In the summer of 2012 he was named one of the 'Stars of Tomorrow' by Screen International. Gravelle filmed Mr Selfridge between April and October, and the series débuted in the United Kingdom in January 2013.
In 2016 Gravelle played the young Paul Finchley in the Channel 4 drama National Treasure.
