Additional Sessions and District Judge Mohammad Gulzar passed the order after the case was heard on a day-to-day basis, starting September 27.
Ghandy (66) told reporters after the acquittal that he had disassociated himself from the banned outfit of CPI(Maoist).
The prosecution had a list of 13 witnesses, out of which one could not be produced and was shown as untraceable.
Ghandy was booked in January 2010 by Patiala Sadar police under sections 10, 13, 18 and 20 of the Unlawful Activities (Prevention) Act (UAPA) and sections 419 and 120-B of the IPC for holding meetings on the premises of Punjabi University in April and May 2009, in an alleged bid to promote Maoist ideology.
He was brought here from Cherlapally Central Jail, Telangana, where he was lodged on September 27. The case was concluded in 20 days as the hearing was held on a day-to-day basis.
Earlier in May, one of Ghandy's associates, Bacha Yadav who was also facing charges under UAPA was discharged by the same court for want of prosecution sanction, which was to come from the Union government.
Yadav was released from the Central Jail, Patiala, in June.
Ghandy had pleaded that he was booked for delivering the lecture in Aoril and May 2009 and at that time, CPI (Maoist) was not banned. The organisation was banned in June 22, 2009, he had argued.