As it stands now, Kurt, we don't know what his death was due to. AFAIK, an official autopsy report has yet to be released and we don't know if it also involved a forensic coroner's assessment, toxicology, and anything else that would be relevant. What we have are leaks devoid of any context and partial information provided by parties on both sides as well as what appears to be factual reporting. For example...
Attorneys for the Gray family have stated his spine was severed.
Another person who was apprehended and also riding in the van but separated by a metal wall stated he heard thrashing around. Speculation was that Gray was looking to injure himself. Could drugs in his body caused a psychotic behavior?
Gray had recently won a case from an auto insurance company for I believe personal injury but the award was a structured payment. He was looking to convert that into a lump sum.
I haven't seen it confirmed but read that he had recently had spinal surgery. What type, how recently, post operative care, admonitions, I don't know.
There's a lot we don't know and when it's finally pieced together hopefully we'll figure it out.
But there are things I'm pretty certain of. People wound up destroying various parts of their community. Student who were let of school immediately went on the offensive. Change Freddie Gray to a white guy and nothing happens. Media outlets and personalities have and will continue to frame things in a manner consistent with their preferred narrative.