Is gray matter disease the same as 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).

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.