Zelalem emigrated to the United States with his father in 2006, following his mother’s death in the previous year, and settled in Washington, D.C.. He played with several teams: first with MSC United, Bethesda Soccer Club and BNC Revolution, before joining the elite club team Olney Rangers and the varsity team at Walter Johnson High School in Montgomery County, Maryland.

Zelalem began training with the United States under-15 side in 2012, but was unable to play in matches at that time due to being classified as a permanent resident, but not yet a U.S. citizen. In May 2014, it was reported that Zelalem’s father, Zelalem Wolydes, held U.S. permanent residency and intended to apply for U.S. citizenship. If Wolydes gained U.S. citizenship before his son turned 18, then under the Child Citizenship Act of 2000, Gedion would also acquire U.S. citizenship and become eligible to represent the United States internationally. He would also be able to retain his German passport which allows him to live and work in the European Union without restrictions.

Arsenal scout Danny Karbassiyoon discovered Zelalem in the United States playing for Olney Rangers during a Dallas Cup match. After contacting his coaches, Zelalem was flown to London for summer training with the Arsenal youth academy; following the end of this camp, he was offered a permanent place in the Academy. Zelalem started out playing in Arsenal’s under-16s squad, before graduating into the under-21s in April 2013. His début for the under-21 side came against Liverpool U21 that month, but was unable to prevent Arsenal slipping to a 3–2 defeat at Anfield; however, he drew praise for his performance. He made his second appearance for the side less than a week later in a 3–2 victory over Wolverhampton Wanderers U21, once again drawing praise for his performance.

In July 2013, Zelalem was included in Arsenal’s 24-man squad for their Asia tour. Although regarded as a surprise inclusion, strong performances against an Indonesia Dream Team, the Vietnam national team and Japanese side Nagoya Grampus led to him being touted as a future star, and being compared favorably to Cesc Fàbregas.

Prior to making his one-time switch to the United States, Zelalem was eligible to represent Ethiopia internationally, due to his parents’ nationality. In October 2013, Ethiopia’s coach Sewnet Bishaw and Ethiopian foreign minister Tedros Adhanom stated that they hoped Zelalem would opt to play for Ethiopia, but Zelalem turned down the opportunity.

Before becoming an American citizen, Zelalem had previously played for Germany’s youth teams. He played for Germany’s under-15 and under-16 sides, making two appearances for the under-15s and eight appearances for the under-16s. In November 2013, Zelalem represented Germany at under-17 level in a friendly versus Spain. But in March 2014, he withdrew from the Germany under-17 national team for the Group 5 2014 UEFA European Under-17 Football Championship elite round fixtures, and again turned down the Germany under-18 national team for friendly matches versus France in March 2015, citing his desire to represent the United States instead.

On January 24, 2014, Zelalem made his debut with Arsenal’s senior squad, coming on as a substitute for Alex Oxlade-Chamberlain in the 71st minute of their 4–0 home FA Cup victory against Coventry City. And later that year, on March 18, 2014, Zelalem signed a contract extension with Arsenal to last until 2017. Zelalem soon made his second appearance for Arsenal, coming on as a substitute in the beginning of the second half, in a 4–1 away win against Galatasaray in UEFA Champions League on December 9, 2014.

The Washington Post reported that Zelalem became a U.S. citizen on December 29, 2014, and that Zelalem had applied for an expedited U.S. passport. After Zelalem received his citizenship, Sunil Gulati, head of U.S. Soccer, revealed that the board for U.S. Soccer had started the process to allow Zelalem to become eligible to play for the U.S. national team. Due to a FIFA rule designed to prevent players from naturalizing to and representing nations to which they lack ties, Zelalem was subject to waiting a minimum of five years before he could start to represent the United States. Gulati stated U.S. Soccer filed for an exception, since Zelalem attended middle school in the United States. He was approved to play for the United States and was added to the United States squad for the 2015 FIFA U-20 World Cup on May 13, 2015. He made his debut against Australia in a U-20 friendly match on May 19, 2015, which the United States won 2–1.

On August 24, 2015, Zelalem was loaned to Rangers until January. He made his debut in a 5–0 win over Airdrieonians in the Scottish League Cup, setting up goals for Martyn Waghorn and James Tavernier. He made his league debut in Rangers’ Scottish Championship game against Queen of the South which Rangers won 5–1. In January 2016 his loan was extended through to the end of the season.

On January 24, 2017, Zelalem signed a contract extension with Arsenal and accepted a loan move to Dutch second-tier club VVV-Venlo, lasting until the end of the season. He made nine league appearances as VVV Venlo won the 2016-17 Eerste Divisie title.

After no first team appearances at Arsenal upon returning in 2017, Zelalem joined MLS club Sporting Kansas City for free on March 10, 2019.

Gedion currently lives in the United States where he is pursuing his professional career. In July 2018, Gedion lost his younger sister, who was 10 years old at the time, to a tragic car accident during a family visit to Ethiopia. According to family members, this tragic accident had a big impact on Gedion, but he has shown great determination to overcome the grief.

After Zelalem’s option was not picked up by Sporting Kansas City, he signed for fellow Major League Soccer side New York City FC on January 11, 2020.