Is gray matter disease the same as dementia?

Yes and no. Dementia can affect both gray matter (non-myelinated neurons) and white matter (myelinated neurons). Many different disease processes affect both gray and white matter that do not result in a formal diagnosis of dementia.
Not exactly. Various types of dementia affects grey mater or white mater or both. Depending on type of disease, reversibility, extent location may affect the intellect (dementia).