"I'd like both of them to show their numbers. CBS is a loaded laggard," the billionaire value investor said on "Fast Money Halftime Report."
"Viacom is an even doubly loaded laggard," added Gabelli, the second-largest holder of Viacom voting shares.
However, the Gamco founder refused to be pinned down on how he would vote if a deal were to be brought to the board. Gabelli's investment firm has nearly $40.7 billion in assets under management.
National Amusements Inc., which controls the voting shares of Viacom and CBS, has twice tried, and failed, to bring the two media companies back together again to gain scale in a landscape of megadeals galore this year. CBS and Viacom had been under the same umbrella from 2000 to 2006 before they were split into separate publicly traded companies.
Joseph Ianniello, who had been chief operating officer at the network, took over as acting CEO, while the board started a search for a permanent successor.
Meanwhile, CBS and NAI, the holding company of Shari Redstone and her family, also agreed to drop their lawsuits against each other, with a stipulation that NAI and the Redstones won't pursue a CBS-Viacom deal for at least two years.
Still, that does not prevent the companies from negotiating a deal independently.
Days after Moonves' departure, Viacom CEO Bob Bakish told CNBC that shareholders should focus on his turnaround plans for the media company, not on deal speculation.
Some analysts say a merger with CBS could be more likely with Moonves gone.
"There's no question that scale is valuable," Bakish acknowledged at the time. But he added, "There are a lot of ways to get the benefits of scale. We're getting them more and more inside the company."
On Friday, Gabelli also said on CNBC, "We're going to tell Shari Redstone: Be the same as the owner, Bob Kraft, of the [New England Patriots]." Using a football analogy, he said, "Stick with your QBs."
"Ianniello is going to follow through on the strategic plan [at CBS]. And Bob Bakish is going to do a great job at Viacom," Gabelli added. "I'm a cheerleader for Shari."