[7] At age nine, he was accepted as a pupil by Alexei Mishin in Saint Petersburg but was coached mainly by his wife, Tatiana Mishina, for the first few years.

He won the gold medal at the junior level at the 2007 Coupe de Nice and was placed 9th at the 2008 Russian Championships.

Competing in the 2008–2009 ISU Junior Grand Prix, Gachinski won silver at the Spanish event and was placed fourth in Great Britain, thus qualifying for the 2008–2009 ISU Junior Grand Prix Final, where he finished 8th again.

He won gold in Belarus and silver in Germany, qualifying him for the 2009–2010 ISU Junior Grand Prix Final, in which he was placed 6th.

In 6th after the short program at the 2010 World Junior Championships, Gachinski earned enough points in the free skate to take the bronze, despite popping his planned quad.

Gachinski won his first senior national medal, silver, at the 2011 Russian Championships behind Konstantin Menshov.

In preparation for the 2011–2012 season, Gachinski took part in Mishin's training camps in Jaca (Spain), Tartu (Estonia), and Pinzolo (Italy) and worked with Stéphane Lambiel.

Gachinski won the silver medal behind teammate Evgeni Plushenko, who is also coached in Saint Petersburg by Alexei Mishin.

After being placed eighth at the 2013 Skate America and sixth at the 2013 Rostelecom Cup, he finished sixth at Russian nationals and was not included in the Russian team to the European Championships, Winter Olympics, or World Championships.

Gachinski moved to Moscow on 6 January 2014 and began working with Tatiana Tarasova, Alexander Uspenski and Maxim Zavozin.