I don't know what you mean by "TfL ticket area" but daily caps absolutely apply to Gatwick - it's £37.30 peak and £23.70 off-peak.
Can you share where you found these prices please? I was looking for similar re Harpenden the other day and couldn't find. The TfL website just has the annoying 'interactive' price cap finder which only includes zones 1-9. As with the single fare finder, it's frustrating they don't publish proper fare tables any more which are much easier to read through.