According to the Park Service, press release, Swiftcurrent Motor Inn (and I presume the restaurant as well) should have closed on Sept. 12 and the campstore will close on Sept. 20. You should be able to get a beer and meals (for those who don't consider beer to be a meal) at Many Glacier Hotel as long as it is open.
http://www.nps.gov/glac/parknews/news09-60.htm
A call to GPI should confirm these closing dates as well as the hours these places are open. In the off season the campstores tend to close early as my daughter and I noticed when we were there around June 10th a few years ago. Don't expect to finish a hike at 7:30 P.M. and stop at the campstore to buy some gatorade!
I have no idea what the closing dates are for some of the outside-the-Park places such as the Firehouse restaurant in Babb (possibly open all year if it survived the summer), the Babb Press, Two Sisters, Park Cafe, Johnson's Cafe, Cattle Baron, etc. or the places in East Glacier (except for the Two Medicine Grill which is open all year). Maybe someone can fill us in on these details.
If you go I hope you will give us a report on what is open and what is not. Some of us do have a fear of off-season starvation.
Lyman