Career
Gero started out playing football in the streets of Kano before signings his first professional contract with El-Kanemi Warriors at age sixteen. The next year he moved on to Lobi Stars. During the midseason transfer window in April 2012 Gero moved to Enugu Rangers from Kaduna United F.C. and scored instantly on his debut in against Akwa United which helped his team to a valuable 1-1 draw that kept them in touching distance of winning the 2012 Nigeria Premier League.
In August 2013, Gero signed a contract with Osters, a team relegated to the Superettan for the 2014 season (the 2nd highest league in Sweden).
In December 2014, Danish club Viborg FF announced that they signed Alhaji on a free transfer. He played 15 matches and scored 1 goal in his first season.
16 January 2016, Viborg FF and Gero agreed to terminate the contract after minimal playing time. Gero signed with Östersunds FK on 18 January 2016.
Gero made his debut for the Nigeria national under-20 football team at the 8 nation tournament in South Africa.
Gero also played for the U20 team in the 2013 Fédération internationale de football association U-20 World Cup. In July 2013 he was selected for the Nigeria national football team squad that played against the Ivory Coast national football team