Mystery Shotgun20-vear-old son of a San Fran- C *man who shot him down at wPolice* sought a motive to tl the shotgun blasting of Robert p Gross, who was hit in the groin shortly before midnight seconds si after opening the front door of hi his home in response to a ring- elt; ing doorbell. trOne of two men standing there tishot him with a sawed-off shot- a, gun. he told police. Gross, the ; * son of Dr. Alvin J. Gross, said 11! he had never seen either man abefore.Gross was taken to St. Francis o Memorial Hospital for emer- tl gency surgery. He was still on w the operating table hours after wBlast Wounds Doctor's Sonhithhlt;cisco physician was critically E wounded last night by a gun- c(point-blank range.tho attackifThe victim’s sister said she e saw a red convertible speed i fawav from the houslt;RTelevision Cable (