In August 2015, Gumennik debuted on the ISU Junior Grand Prix (JGP) series, coached by Oleg Tataurov, Tatiana Mishina, and Alexei Mishin.

[4] Coached by Daineko and Vladislav Sesganov,[1][5] Gumennik started his season with a gold medal at the 2018 JGP event in Vancouver, Canada.

He ranked fourth in the short program but won the free skate and outscored the silver medalist, Tomoki Hiwatashi, by a margin of about seven points.

[8] In February 2019, Gumennik won the silver medal at the 2019 Russian Junior Championships after placing first in the short program and fourth in the free skate.

[8] In the 2019–2020 season, Petr Gumennik remained on the Russian junior figure skating team, coached by Veronika Dainek, Alexander Ustinov and choreographer Nikolai Moroshkin.

[12] Beginning the season on the 2019–20 Junior Grand Prix, Gumennik won the gold medal at the event in Chelyabinsk, Russia.

Qualifying for the Junior Grand Prix Final, he placed fourth in the short program after falling on his opening triple Axel.

[17] Gumennik overslept and missed the morning practice session for the free skate, where he came tenth with errors on his quad Salchow attempts and dropped to seventh overall.

[24] With the COVID-19 pandemic continuing to affect international travel, the ISU opted to run the Grand Prix based primarily on geographic location, and Gumennik was assigned to the 2020 Rostelecom Cup.

[29] Gumennik won the 2021 CS Denis Ten Memorial Challenge to open the season, setting new personal bests.

[30] His first Grand Prix assignment was initially the 2021 Cup of China, but following its cancellation, he was reassigned to the 2021 Gran Premio d'Italia in Turin.