As reported by Censor.NET, this was announced by war reporter Andrii Tsaplienko on Facebook.
Tsaplienko says Gratov is a friend of Donbas terrorists Motorola (late Arsen Pavlov), Givi (late Mikhail Tolstykh), and leader of the so-called "Donetsk People's Republic" Alexander Zakharchenko. He was detained with a fake Ukrainian passport for Valerii Voitenko, "a modest retiree," as put by Tsaplienko.
After crossing the border with unrecognized Transnistria, Gratov was to become a "real Russian colonel" Gratov, former chief of security of Transnistria president, acting instructor for Donbas militants. Gratov has been helping Donbas terrorists since 2014.
"He still thinks he overacted at the border, when leaving Ukraine and entering Transnistria. But in fact, the counterintelligence has long known that "Valerii Voitenko" is actually certain "Sanych," known in Moscow as airborne Colonel Valery Gratov.
Tsaplienko says Gratov was trying to act as a figurehead, as if he was doing nothing in the Donbas.
But in fact, he has been instructing Donbas separatists since early 2014, when the war hadn't yet begun, Tsaplienko reported and provided copies of instructions written by Gratov for events in spring 2014, when the power in the Donbas was seized by separatists supported by Russia.