most of them in the 90s and beginning of the 2000s, but the French had their best players in the 80s and in recent years, precisely eras with several all-time greats.
I think Grosjean had a great shot at the beginning of 2000's also but he was quite unlucky to face Federer at Wimbledon when he just peaked and Agassi was also unplayable in this AO 2001.
As you said, no French players have had all the weapons at the same time. Monfils is probably the more completed player we've ever had. But he has other problems : injuries all the time, no tactic sometimes, lost often his focus, used to choke especially in the big match (look his final record)... Grosjean with a better BH would have been outstanding I guess.