According to a report in The Information on 19 July, Apple has confirmed both the departures.
One of the original Siri co-founders, Gruber "will be pursuing personal interests in photography and ocean conservation."
Prakash joined Apple in 2013 after Apple acquired the search engine and analytics firm Topsy where he served as a CEO.
"The deal was valued at upwards of $224 million. Apple shut down Topsy two years after its acquisition," reports 9to5mac.
Giannandrea will look after further development of Core ML and Siri technologies.
According to media reports, Apple lags behind from Google and Amazon in key AI areas like Natural Language Processing (NLP) and computer vision needed to power voice assistant features in Siri.
Apple has invested in Siri and HomeKit to compete with Google Home and Alexa-based smart home systems.
In 2017, Apple added an AI chip called Neural Engine to the A11 Bionic for the iPhone 8, iPhone 8 Plus and iPhone X to work with the Image Signal Processor (ISP).