Grero is a red grape variety found in Umbria in its native Italy.
Viticulture: Giampaolo Tabarrini told me Grero is fairly easy to grow as the bunches are not too compact, the only worry being susceptible to peronospora (downy mildew).
Wineries
See: Tabarrini (Montefalco).
Bibliography
Italian Wine Unplugged (Positive Press, 2017), p.254.