I'd practice recall with a long line first, and have your pockets stuffed with really smelly, interesting treats, but wouldn't let him off unless your very sure he'll come back.
Our Murphy is very well behaved off the lead, he was let off the lead quite quickly. He run's on ahead, and then stops, looks for us and either runs back without being called, or waits for us. Misty on the other hand is not good off the lead, especially if she's on the sent of something, so she's only off if were well away from traffic, like the beach or the forest.
Personally if by gentle leader you mean the "Halti" I wouldn't use it on a Cavalier. They have such short muzzles, and it can easily end up irritating their eyes. I used to see a poor Tri dragged along beside a pram, the Halti was always sitting right along it's eyes and it could hardly see where it was going. That's just my two cents, but I feel a front clip Sense-ible Harness is much better.
Paula - mum to Murphy(6) & Misty(7), and Jerry our cat.