Indeed, you must have a flat left wrist!
One of the things I changed in my swing when learning TGM was moving my grip to 10-2-B from a 'classic' Hogan style grip. It is perhaps the single biggest change I have made. I still slip out of it if I don't monitor my alignments well at address and get the right hand grip too far down in the fingers and/or give up the 'gap' in my left hand/thumb.
No discussion of TGM would be complete without a focus on learning the Flying Wedges, and really 'getting' your grip correct is the key.
IF you learn to set the wedges as you address the ball, to take your grip at impact fix, and to use extensor action the mystery of 'to roll or not to roll' will in many ways take care of itself, as will the flat left wrist.
The wedges will 'lock in' your alignments so that you can 'do less' during your swing.
Jim's warning is indeed a valid one, but the root cause of that breakdown is more than likely an improper grip and/or improperly set wedges.
The thing about swinging is that you can 'get away' with a lot of sloppy areas of your motion, because CF will take care of quite a lot. Learning 'full roll' by overdoing it is a key first step in taking advantage of CF to align the clubface/shaft/head for impact. The next step is really understanding the wedges, 10-2-B and extensor action.
In fact those three things are a really great shortcut to learning TGM
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"Support the On Plane Swinging Force in Balance"
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Simplicity buffs, see 5-0, 1-L, 2-0 A and B 10-2-B, 4-D, 6B-1D,
6-B-3-0-1, 6-C-1, 6-E-2