Giuly put PSG ahead and scored again in the second half having twice been set up by Peguy Luyindula, who also netted by flicking the ball past keeper Jerome Alonzo after 20 minutes.
Djamel Abdoun made it 2-1 before the break with a penalty to reduce the arrears but PSG midfielder Stephane Sessegnon's powerful drive 20 minutes from time completed the rout.
Third-placed PSG now have 42 points from 23 games, level with second-placed Girondins Bordeaux who travel to Olympique Marseille, who are sixth with 38, on Sunday.
Seven times champions Lyon, who have been struggling this year, visit Nice, also on Sunday.
At Nantes's Stade de la Beaujoire, former Barcelona winger Giuly, who started in place of the injured Guillaume Hoarau, produced a man-of-the-match performance.
He poked the ball home from Luyindula's cross in the 11th minute and doubled his tally three minutes after the break when he collected a defence-splitting pass from the striker.
Toulouse rose to fourth on 41 points with a 2-0 win at home to Le Mans thanks to Moussa Sissoko's strike before the break and Andre-Pierre Gignac's 14th this season late on.
Lille, who were 2-0 down at halftime, moved up to fifth when second-half goals by Pierre-Alain Frau, Robert Vittek and Eden Hazard helped them to a 3-2 win over strugglers Sochaux.
Seventh-placed Stade Rennes lost 1-0 at bottom team Le Havre, who grabbed a last minute goal through Amadou Alassane.
Normandy side Le Havre are nine points away from the safety zone after 17th-placed St Etienne beat visitors Caen 3-2.
