As a youth player Gatis Kalniņš played for his local club FK Valmiera, being taken to the first team in 2000.
After scoring 3 goals in the first 8 matches he decided to leave and went abroad, signing a contract in Cyprus with Othellos Athienou from the Cypriot Second Division.
In March 2011 Kalniņš went on trial with the Finnish team Inter Turku, but didn't sign a contract then.
Before the start of the 2012 season Gatis joined the Latvian Higher League newcomers FS METTA/Latvijas Universitāte.
[3] On 17 November 2013 Kalniņš scored a hat-trick in the promotion/relegation play-offs and helped his team retain a place in the Latvian Higher League, with a 5–2 victory over FB Gulbene on aggregate.