It is understood that Trader Media management has held initial talks with a view to recruiting banks to advise on the deal, but an appointment is not imminent.
The review could lead to a sale or flotation of the business, which was last concretely valued in 2007 at £1.35bn when GMG sold a 49.9% stake to Apax for £675m.
Trader Media Group owns the Auto Trader classified car advertising magazine and website, and has become a predominantly digital business.
According to Trader Media's latest financial results the business generated revenues of £250.9m in the year to 28 March 2010, down 10.3% year on year.
Digital revenue grew 7.8% to £140.3m, though the company made a pre-tax loss of £505.3m after an impairment charge of £463m was applied to its UK titles, reflecting the effect of advertising spend migrating online.
A spokesman for GMG said: "Trader Media Group is a very strong business and well advanced in its digital transition, which makes it a valuable asset for GMG. No decisions have been made about either the timing or nature of our exit from this investment."
GMG took control of Trader Media Group in 2003, paying £600m for the 52% majority stake, valuing the group at £1.1bn.
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