McDonald rejoined the club on a two-year deal in July 2013 from League One side Coventry City, having helped the Gills' 2012/13 title-winning side during a brief but prolific loan spell.
The striker became a firm favourite at Priestfield during the 2010/11 season when, during a loan spell from Norwich City, he scored 25 goals in 43 appearances (three of which made up an impressive hat-trick in a 5-1 victory away to Stockport County). Deservedly, he won five of the six trophies at the club's Player of the Year awards that season.
In August 2011 he moved from Norwich to Coventry for an undisclosed fee, believed to be in the region of £450,000, for whom he scored eight goals in 47 starts and substitute appearances.
Then manager Martin Allen brought him back to the Gills in January 2013 for a one-month loan, in which he scored four times in seven games, and then signed him permanently that summer. Among his memorable goals are an overhead kick from the edge of the penalty box against Walsall in December 2013, an injury time winner against league leaders Wolverhampton Wanderers live on Sky Sports in January 2014 and a powerful free-kick from distance in the Gills' 3-2 defeat to Shrewsbury Town in April 2016.
McDonald started his career with hometown club Witham Town aged 16 and went on to play for Maldon Town and Dartford before earning a move to Carrow Road in 2009.