Demisexual is about when sexual attraction happens - only after a strong emotional bond forms, not with strangers or on first meeting. See the full Demisexual page.
Graysexual (or gray-asexual) is about how much or how often - occupying the gray area between asexual and allosexual, with attraction that's rare, low-intensity, or circumstantial. It's discussed in full on the Asexual Spectrum page.
Both sit on the asexual spectrum and aren't mutually exclusive - someone can experience attraction that's both infrequent (gray) and bond-dependent (demi). They're separate microlabels, each with its own flag, because they describe different mechanisms of attraction, not different intensities of the same thing.