We have used GCT before on land and Cruises. You will not be disappointed. They tend to your every need. GCT and OAT are both good. We used GCT on a 28 day cruise from Istanbul to Capetown. Had a wonderful time.
On your Med cruise: all the ports are wonderful. While In Barcelona I recommend using the City bus with an upper deck. You pay for the ride and can get off anywhere , sight see, then get on the next bus. It is over 4 hour ride. Enjoyed it very much.
If you go into Cannes, France try to go to Monte Carlo by train. Walk to the Train station in Cannes and ride for about an hour to Monte Carlo. Worth the trip. Heck of a lot cheaper than taking the Shore Excursion from the ship. Be sure you have your ticket validated before you board the train. If you dont they will fine you.