Tamaș started out as a senior at FC Brașov in the 1998–99 season, and was a journeyman who competed professionally in Turkey, Russia, Spain, France, England, and Israel, apart from his own country.

[4][5][6] He began his senior career in the 1998–99 Divizia B season with FC Brașov, making his debut when coach Cornel Țălnar sent him on the field for the last minutes of a league game with Sportul Studențesc București.

[4][8][14] He spent half of season with The Gladiators, playing constantly in the defense with compatriots Adrian Iencsi and Florin Șoavă until January 2005 when he was loaned back to Romania at Dinamo because Spartak bought Nemanja Vidić on his place.

[4][6][18] After Spartak sold Vidić to Manchester United, he returned to play for them for a short while, afterwards being loaned again, this time at Celta Vigo where he was wanted by coach Fernando Vázquez for the 2006–07 season.

[4][20] At the start of the following campaign, Tamaș was bought by Ligue 1 club Auxerre for €3 million, there he worked with coach Jean Fernandez, being teammate with Romanians Daniel Niculae and Vlad Munteanu.

[29] He made his Premier League debut in a severe 6–0 away loss in front of Chelsea on 14 August, but overall he had an appreciated performance in his first top-flight season, at one point the club giving him a raise, manager Di Matteo stating: "Gabi is an important figure at West Bromwich.

[41] During the first half of the season, Tamaș only appeared for the development squad and wasn't involved in the first-team trainings starting from November, resulting in his frustration and claiming he wanted to leave the Welsh capital.

[6][50] In the beginning of the 2018–19 season, he won the Super Cup, equalizing in the 28th minute of the 1–1 with Hapoel Be'er Sheva, then he scored with a panenka at the penalty shoot-out which ended 5–4 in favor of his side.

[52] After about one year spent in Liga II at Concordia Chiajna, in October 2023 at age 39, Tamaș suddenly decided to retire saying that he could go on physically but not psychologically, claiming disappointment with the level of Romanian football, also saying that Dinamo was the club he liked the most playing for.

[5][33][53] Tamaș was handed his Romania national team debut by Anghel Iordănescu on 12 February 2003, coming on as a substitute for Cosmin Contra in the 90+1st minute of a 2–1 friendly defeat of Slovakia.

[54][60][61] Tamaș was known for his controversial lifestyle outside the football pitch, his numerous nights of drinking out and provoking fights or material damages often making the headlines of Romanian media.

[64][65][66] On 11 August that year, Tamaș and his teammate Adrian Mutu were excluded from the Romania squad after they were caught drinking in a bar the previous night, while the national team was preparing for a friendly match against San Marino.

[63][72] One week later, he signed a one-year contract with CFR Cluj, but the club dismissed him shortly after he and his teammate Florin Costea caused an incident in a local pub.

[74] Also in the same year, while waiting in line at a pizzeria from Doncaster, he got punched by someone for no reason below his right eye, his zygomatic bone being broken, having to get a surgery, then for a while he wore a mask during training sessions and games.

[76] His first spell at Steaua ended when he was dismissed after missing a training session following a night out; manager Mirel Rădoi stated that Tamaș lied to him by claiming that his phone alarm did not ring.

[79] In October 2022, whilst drinking along with several Petrolul Ploiești teammates at a restaurant, Tamaș called their manager Nicolae Constantin on his phone and criticised his tactics.