Whittingham was at Aston Villa when Gregory was part of the coaching staff in 1994, and the former Villa forward has revealed that the new Crawley boss once paid for him to leave the army so that he could pursue a professional football career.
The 49-year-old told the club's official website: "One of the attractions was working with John. I guess that I owe my professional career to him because he paid £450 to buy me out of the army all those years ago.
"A lot has happened since then but I was keen to come in to work with John again. Obviously I have had my own experience of management so I know what it's like but it's great to be here."
Whittingham was sacked as Pompey manager last month.