"I'm not going to talk about the guys that are not here," Belichick said, via NFL Network's James Palmer.
Brady's and Gronkowski's absences for the voluntary OTAs were expected, as previously reported by Palmer and NFL Network Insider Ian Rapoport.
Rapoport, however, reported Monday that both players are expected to attend the team's mandatory minicamp, which runs June 5-7.
Brady has two years remaining on his current deal and will earn a base salary of $14 million in 2018, but desires a new contract.
Gronkowski, who also has two years remaining on his contract, will earn a base salary of $8 million this season.