Gachkar grew up in Overland Park, Kansas and attended Blue Valley West High School. As a junior, he registered 83 tackles, 5 sacks, 2 interceptions (one returned for a touchdown) and 2 forced fumbles. In his last year, he was a two-way player at both running back and linebacker.
Gachkar, rated the number 12 overall prospect in the state of Kansas, chose to play collegiately at the University of Missouri. He was a two-year year starter for the Tigers.
In 2009, he started 13 games at strongside linebacker and had 80 tackles (second on the team). The next year, he was moved to weakside linebacker to replace All-American Sean Weatherspoon, finishing with 84 tackles.
Gachkar was selected by the San Diego Chargers in the seventh round (234th overall) of the 2011 NFL Draft. On July 28, 2011, he was signed to a four-year deal with the Chargers. As a rookie, he played mainly on special teams and posted 12 special teams tackles (second on the team).
He became a valuable special teams player, while also being a solid reserve linebacker. In 2012, he recorded 13 tackles (2 for loss), one sack and eight special teams tackles. The next season, he started 3 games, making 25 tackles (one for loss) and 8 special teams tackles.
In 2014, he received more playing time due to injuries and defensive coordinator John Pagano's rotations at linebacker. He registered 5 starts, 46 tackles (6 for loss), 16 special teams tackles and also scored his first NFL touchdown against the St. Louis Rams on a fumble recovery.
On March 15, 2015, he signed with the Dallas Cowboys a two-year, $5.5 million contract, reuniting with former Chargers special teams coordinator Rich Bisaccia. As a reserve linebacker, he showed the ability to play all three positions and made Jasper Brinkley expendable. He started against the New Orleans Saints and posted 6 tackles. In the eighth game of the season against the Philadelphia Eagles, he saw extended time in place of Sean Lee who was out with a concussion, while tallying 4 tackles (one for loss). He played in every game, registering 8 special teams tackles (tied for fourth on the team) and 13 defensive tackles.
In 2016, he remained a core special teams player and tied for fourth on special teams tackles (7). In the season finale against the Philadelphia Eagles, the Cowboys rested players for the playoffs and Gachkar saw extended playing time, posting 5 tackles (one for loss), 2 quarterback pressures and a half-sack. He wasn't re-signed after the season.
On August 13, 2017, Gachkar signed with the Jacksonville Jaguars. He was released on September 2, 2017.
Gachkar overcame a life-threatening ailment to even be able to play football. In 2007, after his freshman year at Missouri, he was diagnosed with Thoracic Outlet Syndrome. He had developed a blood clot because his collarbone and rib cage were too close together to allow proper blood flow through his veins. As a result, he underwent major surgery to remove one of his ribs, and a second surgery after another clot developed. Gachkar spent more than 20 hours in surgery and nearly a month in the hospital and he lost 30 pounds. The surgeries initially put his football future in doubt, but Gachkar was diligent in his rehabilitation and he made it back for the 2008 season without missing a game.
Gachkar married Lauren Nuckolls, a former Missouri Tiger volleyball player and his high school sweetheart, on March 23, 2013.