Easy. An autopsy would be done in order to determine cause of death. Look up when an autopsies are done in NJ, and you'll understand why.
Part of an autopsy would have been blood tests, to see if the Gaudreaus had any drugs/alcohol, etc in their system, because their death is part of a criminal investigation and that information is relevant to that investigation.
If the prosecution withheld information from the defense for whatever reason (wanted to nail this guy, wanted to protect the Gaudreaus reputation, didn't think their BAC was relevant, or whatever their motivation), that's a potentially huge failure of discovery of evidence.
Also to anyone saying the Gaudreaus BAC isn't relevant, ...if the prosecution had this information before the indictment (they had Higgins BAC), so they probably had the Gaudreaus. They have a legal responsibility to share that information. It's not up to the prosecution decide what information is or isn't relevant before going infront of a grand jury... That's the courts and grand jurys job. To decide what is relevant, and if there's enough evidence to go to a trial.