I have found intrinsic motivation works best for bad habits, and really for good habits too. Not much management there, and sometimes not.much GTD either. Suppose I want to eat a healthier diet. When shopping, I buy fewer processed foods, less fatty foods, more fresh vegetables, et cetera. I feel better, which establishes a positive feedback loop. My tastes change: less fast food, more fresh healthy food. Now a couple of caveats: I shop at a store which has a lot of interesting and healthy food, and I can absorb the cost of eating more healthy food. I am an experienced cook, and I continue to expand my abilities there. As you should see, this is a very favorable example for easy change over time (and happens to be factually the case). Possible major obstaclues, like availability of healthier food, cost of food, and changes in cooking presented no difficulties. If they had, then there would have been projects and next actions to overcome those specific issues: find better places to shop, re-allocate budget, learn to cook vegetables, et cetera. Absent those barriers, the process can be gradual and natural if the motivation is there.
Recognizing the barriers and dealing with them is where GTD is valuable. If you want to look at it from a more top-down point of view, you might have a life goal (Level 5) to be as healthy as possible throughout your life. This might give rise to various projects and next actions in Areas of Focus like Health and Household. However, if you lack motivation, you will have problems, even if the path is forward is very easy.