All of my dogs have passed the CGC it is quite easy and I do think it is a good thing to do. There is nothing wrong with training at home, I have done this as well. In fact, the local kennel club has a class that teaches it but I found I could do better on my own.
Funny story about a dog that I had many many years ago. It was a 140lb male Rottie that was the most Teddy Bear like dog you can imagine. He was a total love and a very very easy going guy. There were tons of people in the test area at a training center where they were holding the CGC. They were actually testing many dogs at the same time, each in different test areas. It was our turn and we passed everything with flying colors until the last test area which was the "stay, out of site" In this case we were to bring our dog into a back office with a handler, you could either hand her the leash or tie the dog to a desk and leave the room. The handler was afraid of my dog so I tied him to the desk. NOT good, It was against my better judgment, I should have asked for a handler that wasn't afraid of my dog. Well, he stayed for a little while and then all of a sudden, I am sitting on the bleachers out in the front with about 25 others and we hear this horrible noise. It sounded like a large, heavy wooden desk being dragged down the hall. UGH! My dog had simply gotten up and began walking out to find me. The fearful lady had not shut the door or tried to talk to him at all. He wasn't panicked or anything, he was just board. He still passed because he had stayed well past the time he was required, and he wasn't nervous at all despite the fearful woman in the area with him. Needless to say, everyone got a good laugh.
The testing for this has been different in the past few years. It is usually one dog at a time, the people seem more comfortable around dogs of all sizes and shapes. They even offered the test at a pet fair I was at with Oliver. I didn't know they were doing it, hadn't been working on it but thought, I would try it with him since we were there. He passed with flying colors and even made a few friends while we were there.
Licking is not a bad thing for a CGC, It isn't a great thing for Therapy but if the dog gives one quick lick, it won't be deemed unworthy of therapy work. If your dog is friendly and is used to being handled by you and others, you will likely not have any issues. The biggest thing for many folks is the approaching a friendly stranger with a dog. Practice walking up to people (you know) who have a well behaved dog. Have your dog heel, and then ask to sit before trying to shake the persons hand and then walk past. Don't allow your dog to go by the other dog. It seemed to me that most folks found that to be the hardest part of the test. Also practice leaving your dog with others for a few minutes here and there.