[4] Gu was already selected for the youth national teams as a high school student and participated in the 2006 Cadet (Under 17) World Championships.

The media dubbed him the "scary rookie" after he narrowly defeated compatriot Oh Eun-seok, then world number 2, in the semi-finals of the Asian Games.

[8] Gu won two individual gold medals at the 2010–11 Fencing World Cup and ended the competition ranked number 3 overall.

The Olympics began on a sour note as he, Kim and Won did not make it past the semi-final stage of the individual event.

[13] Gu himself won individual silver in the 2017 worlds and successfully defended his Asian Games gold the following year.

[a][16][17] The 2021–22 World Cup season began uneventfully for Gu as he was not able to participate in the Orléans Grand Prix held in October, having tested positive for COVID-19 the day before the competition.

He and Gu Bon-seung indicated through comments left on one another's official Instagram accounts that they were well aware of the rumors, quipping that it was "an honor" to be mistaken as relatives.