"The fact that he's wearing a beard does not materially interfere with the course of the trial," McGrory told the panel.
Chief Judge Col. William Kern repeatedly asked government attorney Capt. Kenneth Borgnino whether Gross had put his impartiality in question by issuing the order instead of leaving it up to Hasan's chain of command. Hasan hasn't been charged with a grooming violation.
Judge Col. Steven Haight asked Borgnino: "Is it appropriate for the command to virtually punt the forcible shaving issue over to the military judge?"
Borgnino said Gross was merely controlling his courtroom. He said a bearded Hasan at trial would be as offensive to the judge and jury as an obscene signboard.
"This isn't a situation where he's missing a button off his uniform," Borgnino said. Allowing the beard, he said, "would be to cede control of the courtroom to the whims of the accused."
It's unclear when the court will make a decision, which could be appealed to the U.S. Court of Appeals for the Armed Forces.
The Fort Hood rampage was the worst mass shooting ever at a U.S. military installation. Hasan remains jailed.
Angela K. Brown in Fort Worth, Texas, contributed to this report.