That title is about all I know about this topic. I'm looking for extra info on the importance of the French intervention and the consequences, both good and bad, of the maneuver.
Great link here: http://www.france-histoire-esperance.com/marechal-alphonse-juin-vainqueur-du-monte-cassino/
That is a big topic. In order to understand the Gustav Line you could try John Ellis, Monte Cassino or Peter Caddick Adam's Monte Cassino ten armies in Hell then read the Anzio material. For a fictionalised story of the CEF try Days of Glory - AKA "Saving Private Siaid" https://www.youtube.com/watch?v=P45TTGT2XhY&list=PL5mR0PxdNDZ7rlAic_Y5r-4O7LqOqinVn . .