No, the diesel in that truck is not a gas conversion. The gas conversion engine was an Oldsmobile 403 that was reworked into a diesel 350--that was used from '78-'85 in cars and it was available in shortbed 2wd pickups for '80-'81. There was also a 4.3 V8 with that same block and a small bore, and then a V6 4.3 with the same block chopped off.
The 6.2 diesel isn't part of that program. It was designed from ground up by Detroit Diesel to be a diesel for the truck market--showing up in everything from 1/2 tons to 14,000#GVWR step vans and motorhomes. There's nothing wrong with these engines except people treated them like gas engines. You can't let a diesel set for long periods of time or neglect the oil changes. They didn't turbocharge them because it would have added a couple thousand to an already very expensive option. They are pretty weak naturally aspirated--about 130hp/240ft# in LD trim, and 155hp/285ft# in HD trim. The light-duty trim motors are emissions certified for under 8,500# and they use an EGR valve in the center of the intake manifold and pick up exhaust gas from a valve in the left exhaust manifold. That valve can (and will) jam shut and cut your power even more.
The one thing you should know about these motors is that they use the same fuel system that is on the 6.9/7.3 Ford--the Stanadyne DB2 injection pump. This pump is capable of moving enough fuel for 250hp and 450ft#. Problem is, you need a turbo to get past the 155hp/ 285ft# the HD nastily-asspirated motors produce. Once you turbocharge them, they are fine provided you keep the turbine-inlet temps below 1,100F. Their extremely high compression conpared with an engine meant specifically for turbocharging means their combustion temps can get out of hand quickly. Mine puts out 12psi of boost and has no trouble at all pulling a 10,000# trailer.
Incidently, these motors are still produced by AM General's
General Engine Products division, who took them over in 1999 from GM. They have been made for 27 years. So the idea that they are bad is only held in the minds a a few. They are sold as military, marine and idustrial engines, and you can buy one too through GM as a serivce part. They are not cheap. I believe Penninsular Diesel still make a 400hp version with twin turbos.
The best thing about them is they are all over Ebay as the military pulls them out when they overhaul Hummers. They are cheap compared to Cummins so it's easy to have a diesel truck. The older 6.2/6.5s can't be pumped up to 400hp, but they can get the job done and get great fuel economy doing it--which is why I have a diesel. Maybe someday I'll feel like I need 800ft# of torque and swap in a Cummins.