Gardemeister showed early promise in Finnish domestic rallying, winning the Finnish Rally Championship Group A title (under 2000 cc) in 1997 at the wheel of a Nissan Sunny. He made his WRC debut with SEAT for limited outings in 1998 driving the Ibiza, then became a regular championship contender from 2000 onward in the SEAT Córdoba WRC alongside former world champion Didier Auriol. Notable early results included a third place in New Zealand in 1999 and a fourth on the 2000 Monte Carlo Rally.
SEAT's surprise exit from the World Rally Championship after 2000 left Gardemeister seeking alternatives. He secured two privateer outings in a Peugeot 206 WRC, finishing fifth at Monte Carlo and fourth in Sweden, before being drafted into the Mitsubishi Ralliart team for events in Finland and New Zealand.
These performances caught the attention of Škoda Motorsport, who signed Gardemeister for 2002. He drove the Octavia WRC throughout 2002 and 2003, again partnering Auriol, posting best results of fifth at the 2002 Rally Argentina and the 2003 Rally New Zealand. He stepped up to the Fabia WRC in 2004, achieving seventh at the Rallye Deutschland.
Securing a contract with the BP Ford World Rally Team for 2005 — after both Markko Märtin and François Duval departed — Gardemeister delivered the best season of his WRC career. He began with a second place on the Monte Carlo Rally on debut, followed by a third in Sweden, which temporarily put him atop the World Championship standings. He added a second place at the Acropolis Rally and another second in Corsica before the season's end. Finishing fourth in the final drivers' championship was his career-best WRC result, achieved with double the points of his team-mate Roman Kresta. Despite this, Ford signed Marcus Grönholm and Mikko Hirvonen for 2006, leaving Gardemeister without the factory seat.
Gardemeister returned to privateer status from 2006, competing in a Peugeot 307 WRC and later a Citroën Xsara WRC, recording a third place at the 2006 Monte Carlo Rally. In 2007 he drove a Mitsubishi Lancer WRC05 across five events before joining the new Suzuki World Rally Team for 2008.
With Suzuki, Gardemeister produced Suzuki's first-ever WRC stage win in Japan, finished seventh in New Zealand, and scored the team's best points tally of the season. When Suzuki withdrew from the WRC before the 2009 season, he again lost his drive.
From 2009 he competed sporadically in the Intercontinental Rally Challenge, suffering retirements but winning the prologue of the 2010 IRC Monte Carlo Rally.
In 2011, Gardemeister formed his own outfit, TGS Worldwide OU, initially running a Škoda Fabia S2000 and later Škoda R5 machinery. The team became a development platform for young Finnish talent, with drivers including Teemu Suninen, Kalle Rovanperä, Juuso Nordgren, Eerik Pietarinen, and Pontus Tidemand passing through its ranks. A serious back injury suffered in 2012 during the Silk Way Rally brought his competitive driving largely to an end, though he recovered fully after surgery.
Gardemeister's record of four WRC podiums and a fourth-place championship finish represents one of the stronger careers among drivers who were never quite able to lock down a long-term top-tier factory contract.