The pair played together for Real Sociedad and have remained close friends ever since.
They will be in opposite dugouts tonight at Vicarage Road, and speaking before the game, Emery said of Gracia: "When the possibility opened to sign here, the first coach, the first person in the football world I called was him.
"I asked some questions about the football here, the clubs, Arsenal, and he recommended to me that I come here."
He continued: "For example, we had dinner, their coaches and our coaches, one day in St Albans in September.
"We spoke about football here, English football and their experience because they were here six months before us.