Is there an easy way to find holiday treats that are gluten free?
Visit Erin McKenna's in Disney Springs. I can't say 100% they have holiday treats though. Sprinkles has a GF chocolate chip cookie. Vivioli Gelato has gelato and sorbet, though again, not 100% sure what they will have w/holiday theme. The Ganachery should also have GF sweets, and probably something holiday themed that is also GF.
There should be a holiday option for the Dole Whip. If not, the Poly had a 50th anniversary version that was slightly unique. It was just a dole whip sprinkled with edible shiny glitter, but it was pretty and GF.
I can't say for certain for Christmas, but the Beach Club Marketplace had an array of (packaged) sweets around Easter. Most were not gluten-free, but some were.
It won't help this time, but for a future visit, consider going during one of Epcot's Festival's. Food and Wine runs to 11/20, Art Festival starts 1/13/23. Both print a brochure with icons that list what is gluten-free. Most options are not, but some are.
Looks like Isle of Java has holiday hot cocoa at AK.
Epcot Holiday Festival (11/25-12/30): Chestnuts booth has GF glazed nuts. Holiday Hearth has a GF peppermint bark. Holiday Sweets (near Port of Entry) has a Peppermint sundae. L'Chaim has potato pancakes.
If you get the "Festival of the Holidays" brochure in Epcot, it should list all the GF options at the various booths with locations! There is also a long list of holiday beverages, some w/alcohol and some without alcohol, but I imagine most are GF except the beer.