Correct, GM did own Detroit Diesel and Allison Transmissions at one time. However, the 6.2 has nothing to do with the Oldsmobile 5.7 and was entirely a product of Detroit Diesel, but was made for GM. Detroit Diesel does, indeed, make 4-stroke diesels and has for quite some time. The 2-strokes "Screamin' Jimmy" was the OLD 60 series that had a turbo that blew through a supercharger that was gear driver by the timing gear set. Detroit Diesel on-road division is now a subsidary of Diamler AG.
General Motors introduced the Diesel V8 engines for their C/K pickup trucks in 1982. This engine family, designed by GM division Detroit Diesel, was produced through 2000, when it was replaced by the new Duramax line.
The General Motors light-truck 6.2 and 6.5 L Diesel engines were optional in all 1982 through 2000 Chevrolet and GMC C/K series pickup trucks, Suburbans, Blazer/Jimmy, Tahoe/Yukon, vans, and motor homes.