Just in case you only want personal observation, I think the GCNPis the most fun in winter but, I like winter. I will say that the Canyon is a fairy tale world at any time of year but it is so different in winter it takes on a diferent life. As long as you have the oportunity, it is a don't miss opertunity.
Mather is open year around but they cannot physically plow every camp site. They are old and intended for vehicles of the 1930-40s. Reservations may not be imperative but advisable. Trailer Village is open and roads and sites are cleared and full hook ups.
As far as getting snowed in sure it's possible, elevation is 7100 feet. It's not to likely.
Ti reinterate, v
https://www.nps.gov/grca/planyourvisit/cg-sr.htm
