The deal will broaden Gartner's research business through the addition of CEB's services, which include research and analysis related to human resources, sales, finance and the law.

Gartner is offering $54 in cash and 0.2284 of its shares for each CEB share. The deal represents a premium of about 25 percent to CEB's Wednesday close.

CEB's shares were up 16.4 percent at $72.05 in premarket trading, below the implied offer price of $77.25 per share. Gartner's shares, which closed at $101.79 on Wednesday, were untraded.

Gartner shareholders will own about 91 percent of the combined company.

CEB, headquartered in Arlington, Virginia, has a 35-day "go-shop" period during which it can solicit alternative proposals.

Stamford, Connecticut based-Gartner said the deal would immediately add to adjusted earnings per share on completion, expected in the first half of 2017, and be "double-digit percentage accretive" to adjusted EPS in 2018.

Evercore and Goldman, Sachs & Co advised Gartner. Centerview Partners LLC was lead adviser to CEB, with Allen & Co LLC also advising.

(Reporting by Laharee Chatterjee and Narottam Medhora in Bengaluru; Editing by Ted Kerr)