"Ducon Lajoie" is derived from "Dupont Lajoie". Dupont Lajoie1 is a name derived from a 1975 movie and it personifies a racist cowardly common man.
Deriving Dupont into Ducon makes the person even more stupid. Calling someone ducon in French is an insult, it means you're telling the person they're stupid.
I have never heard "Gratien Bonheur", but you might have misheard and your workmates could have been talking about "Gontran Bohneur", a cartoon character usually known just as Gontran ("Gladstone Gander" in English), Gontran is a lazy unpleasant character who keeps bragging about his good luck.
Even if both depict unpleasant stupid characters there's a difference between calling someone a "Ducon Lajoie" or a "Gontran Bonheur". Calling someone a "Ducon Lajoie" stresses the racist feature. "Gontran Bonheur" is not as common as far as name calling is concerned and it applies to someone who likes boasting and whom you think doesn't really deserves his good luck.
The film is known has The Common Man in the English speaking countries.