This model was trained predominately on real life images with gaps in their teeth, mostly in the front teeth. Higher values tend to give inconsistent and warped teeth, particularly if the shot is not a close up.
I tend to use strengths between 0.5-0.7 - however, depending on your desired teeth you may have success with different strengths. Strength of 1 tends to give very inconsistent results so will probably require multiple generations.
To increase the appearance of gap-teeth, include prompts like: gap tooth, gap teeth, prominent gap between front teeth, teeth exposed, mouth focus, lips parted.