Lawmakers from rival parties subjected Gysi to an hour of accusations in parliament that he had betrayed his clients by revealing their secrets to the authorities in the communist state while working as a lawyer.
Gysi, 60, made a brief statement to deny the accusations and then left the house to applause from his party.
The Left, or Die Linke in German, has staged a meteoric rise since last year with fierce anti-capitalist rhetoric to become Germany's third largest party.
Accusations about Gysi's past are not new but they made the headlines again last week with the coming to light of new documents suggesting that he worked as a so-called "IM", an unofficial informer, in 1979.
"You have been trying for 10 years to besmirch me in an effort to get at my party," Gysi told deputies.
The head of the Stasi archives Marianne Birthler said on German television that it was certain that Gysi "knowingly and voluntarily worked" for the Stasi, citing the case of an artist on whom "only Gregor Gysi" could have informed.