I believe it's open year-round. I've been there end of Nov and mid-Dec and it's been open. I just think the hours might be shortened during that time frame.
Yes, the DisneyLand Hotel and the GC pools are open year round. Yes, they do have the Lifeguards year round as well! Have stayed at both hotels a lot in the last ten/eleven years and the pools are always open, even the Christmas Holidays . The Hot-Tubs are great in the evening after a long day in the parks. Spend about twenty minutes in the Hot-Tub, then swim ten or twelve laps in the pool to help get the kinks out before dinner, Great! ! ! ! !