After two years playing at the Nottingham Forest academy, McDonagh made his professional debut as an 80th-minute substitute during a 2–1 loss against Cardiff City on 29 August 2015. In July 2016, McDonagh joined National League side Wrexham on an initial one-month loan deal as cover for the injured Jordan White. McDonagh showed his delight at the prospect of playing first-team football, claiming the loan would be "a good experience as a young player coming in from Nottingham Forest". McDonagh made his debut for Wrexham as a substitute in place of Shaun Harrad during a 3–2 victory over Guiseley on 9 August 2016, scoring his first senior career goal. Both Harrad and McDonagh scored as Wrexham defeated Bromley 2–1 on the 24 October, with Harrad hailing their partnership up front; "Once you've got one striker committing, the other one gets the drop downs". On the 26 November McDonagh scored his first career brace as Wrexham defeated Forest Green Rovers 3–1. His strike partner Harrad scored Wrexham's other goal of the game. On 3 January 2017 McDonagh returned to Nottingham Forest, having scored five goals in twenty-one appearances for Wrexham following an "outstanding" spell at the Welsh club. On the 21 January McDonagh was again sent out on loan, this time to EFL League Two club Cambridge United. McDonagh made his debut the same day as a 61st-minute substitute during a 1–3 defeat by Mansfield Town On 7 July 2017, McDonagh was one of four academy players at Forest to renew his contract ahead of the 2017-18 season, although the length of that contract was not disclosed by the club. It was confirmed on 27 April 2018 that Gerry had left Nottingham Forest, following the expiry of his contract.
Gerry signed for National League side Aldershot Town on a youth loan on 7 December 2018. McDonagh made his debut the following day, in a 4-1 away defeat to Harrogate Town. On 9 January 2019, Aldershot Town made McDonagh's loan deal permanent and signed him on a free transfer. Gerry made 13 appearances in total for Aldershot Town, and departed the club at the end of the 2018–19 season.
McDonagh was called up by Republic of Ireland U17 in March 2014, and scored on his debut against Austria U17. Gerry went on to gain five caps, scoring one goal.
