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The deal will see Forrester acquire 900 organisations on Giga's books in what Forrester chairman George F Colony said would be an "ideal fit" for his company and its clients.
Gideon Gartner created Giga in 1995, two years after he parted company with the analyst group that bears his name.
He is backing the Forrester buyout, which values Giga shares at $4.75 per share in cash. Forrester hopes to complete the takeover by the end of February.