What do you think about using a bitless bridle on a very green horse?

I have a 4 year old gelding that has 2 months training. The trainer put him in a gag bit. He wasn't really doing anything bad, just not responding really well. One of my friends said that the gag bit will actually make him worse to where I will need something harsh on him at all times. He said for me to use an O ring snaffle. I want control, but gentle control. I wanted to use a bitless bridle, (similar to a hackamore) but am not sure. I have always heard that a bit is needed when they are green, but never was sure why? Is it for getting the horse to bend good when they turn? Please let me know what you think. Thanks.