What is a "gross understatement"?
It's an acknowledgement of something, but in a manner that insultingly underplays its importance or significance.
"The Cossacks were mean to Jews" would be a gross understatement (Cossacks were used by the Tsars to conduct pogroms, large-scale murder and abuse of an unwanted Jewish population in imperial Russia).
Ron Ziegler, Nixon's press secretary, once famously said of the Watergate scandal that "Mistakes were made." This acknowledged the undeniable existence of wrongdoing without claiming or assigning responsibility for it on his administration's behalf.
These would be gross understatements, which acknowledge some basic facts while denying their enormity or significance.