Gerry Armstrong, who supported English club Leeds United as a boy, began his career in Northern Ireland with St Paul’s Swifts. He only started to play football as a teenager when serving a ban from Garlic Football. He subsequently moved on to play for Cromac Albion and Bangor. In November 1975, Gerry moved to England, signing with Tottenham Hotspur for a fee of £25,000. He made his Spurs debut in a 3–1 defeat at Ipswich Town on 21 August 1976, aged 22. He made a total of 84 league appearances for Spurs, scoring 10 goals.
In November 1980 he was signed by Second Division Watford for £250,000. Watford were promoted to the First Division in the 1981-82 season and Gerry scored the club’s first ever goal in the top flight. In August 1983, he moved to Spain with Real Mallorca. Gerry returned to England in August 1985, signing for West Bromwich Albion on a free transfer. In January 1986 he was loaned to Chesterfield, whom he joined permanently in March 1986 until the end of the season. On his debut, Gerry scored for the Spireites in a 3–1 home defeat to Brentford.
Gerry signed for Brighton & Hove Albion on a free transfer in August 1986. Between 1986 and 1989 Gerry made 55 appearances for the Seagulls scoring 7 goals.
In April 1977 Gerry made his debut for the Northern Ireland national team. He played alongside George Best in a 5–0 friendly defeat to West Germany. Five years later, Gerry was selected for the Northern Ireland squad for the 1982 World Cup in Spain. After a 0–0 draw in their opening match against Yugoslavia, Gerry scored the opening goal in the 1–1 draw against Honduras. With Northern Ireland requiring a win to progress to the next stage, Gerry scored a 47th-minute goal against hosts Spain. Northern Ireland even had Mal Donaghy sent off, but held on to win 1–0.
In the next stage, also a group round, Northern Ireland drew 2–2 with Austria, with Gerry scoring in the subsequent 4–1 loss to France. Gerry made a total of six appearances for Northern Ireland in the World Cup.