My parish has had CGS for few years. I went into the training skeptical - I'm now certified in Level 1 (3-6yos) and I am still skeptical.
Basically it works like this:
1) It is meant to be a very controlled environment set up specifically for the needs of that age group.
2) The children are given presentations meant to spark the wonderment of the child. For example, the Good Shepherd presentation has several parts shown one at a time with several weeks in between presentations spread out over three years. It is meant to allow the child to grow in his relationship with Jesus the Good Shepherd - that He will always take care of us, that He will seek us if we're lost, that He invites us to His table at Mass, that He is really present at Mass etc.
3) All this through the child seeing simple presentations and then being allowed to "work" with the wooden sheep and pasture. The work is quiet time with the material meant to allow the Holy Spirit to speak directly to the child.
4) All except practical life presentations have "wondering with the child" parts where parts of the content the presentation is meant to convey are brought forth for the child's wonderment. For example, the catechist may say "I wonder how the lost sheep felt" and the children may respond. The catechist is never to say "the lost sheep felt scared" or make such a definite statement unless it is in the Scripture.
5) If the child has a misunderstanding it is up to the Holy Spirit to work out in His own time
6) The child needn't be able to convey/articulate anything he is learning or how he is growing in relationship with God
It all just looked like playing to me and I never saw any evidence that it was any more. The training really binds the catechist's hands. The catechist is to be very cautious about interfering with the Spirit. Which means if Johnny seems to be playing cops and robbers with the shepherd and sheep the catechist may not be entitled to step in. The catechist is never to tell the child how to "work" with the material.
If the child asks a question the catechist is to respond with an "I wonder" statement. For example, if a child asks "was the shepherd mad" the catechist is to respond "I wonder if the shepherd was mad" in a voice that (to me) sounds like the catechist knows nothing. And the catechist does the "I wonder" thing constantly - seriously in the training it got to the point where we joked that if a child asked "where's the bathroom" you'd have to respond "I wonder where the bathroom is" so as not to cramp the Spirit reveling it to the child.
The child is also free to "work" with any material which he/she has seen the presentation for. Which means she may just go to the same thing every session but that's okay because the Spirit is working. Except that I've seen children gravitate to the "practical life" materials which are nothing more than busy work (meaning time killers - something to keep the child quiet). There are atria out there with little more than practical life material - stuff that is all well and good for a young child to be working on (fine motor skills and such) but have nothing to do with religious education.
There is no way whatsoever to check whether the child is understanding anything. Each presentation has a direct aim but I've seen presentations where that direct aim is not remotely achieved. Again, a catechist can never correct a child. There are geography presentation and if the child is putting things in the wrong spot - even if it's part of a second presentation that builds on the first - the catechist cannot correct the child.
CGS is suppose to be wonderful. There is supposedly evidence that children grow hugely in their relationship with and knowledge of God. But I have never seen any such evidence. I've seen kids messing about. I've seen kids I gauge to be confused and/or too scared/polite to speak up. I've seen catechists get very different responses than they were prepared for and have no clue what to do (for example for a "I am the vine" presentation the children were basically tearing the plant apart rather than holding it gently and understanding that they are connected to God).
Training takes 90+ hours and $600 per catechist per Level. And must be done in order (Level I then II then III) even if you are a 6th grade catechist who never has anything to do with younger kids. The materials are expensive and time consuming because they are meant to be beautiful and hand made. The books for the catechists to read are expensive. There are all sorts of bags, stamps, and such with the Good Shepherd logo for tons of money. It seems like a money maker to me. My diocese approves of the program but has also admitted to me that the group who makes the approval doesn't really know about the program and is basically going off the word of one person.
I don't like it. I think at best it is usually a waste of time and at worst it is an actual hindrance to the child's growth. I am a DRE and I have no intention of introducing it in my (work) parish if only because of the cost. I'm not all for book work but I think this goes too far the other way. The Holy Spirit does amazing things but expecting Him to revel such mysteries without guiding the child as we are entrusted to do just doesn't make sense. In other words - does the Spirit work in the lives of and speak to children? Absolutely. Does the Spirit revel intricate details of a parable to a child simply by having a catechist read the words to them and move little wooden figures? I'm not convinced.
I gave it an honest, prayerful, dedicated try. But I'm simply not convinced even after 90+ hours of training, hours of observation, reading, and discussing.
If there is anything else I can tell you or answer for you, I'd be happy to.
Hope this helps. Christ's peace be with you.