Looked it up on Memory Alfa, and found the quote. It's from 'Ties of Blood and Water'.
"Still calling yourself gul? I'm surprised you haven't promoted yourself back to legate by now."
"I prefer the title 'Gul'; so much more hands-on than Legate. And less pretentious than the other alternatives: President, Emperor, First Minister... Emissary."
"How about Dominion puppet?"
The quote clearly states BACK to legate. That would mean he used to be one. But for the life of me, I can't remember any episode where Dukat is refered to as a legate.