For this dish, you will need a 1 1/2- to 2-quart nonreactive mold. The jelly can be made up to two days ahead.
Lightly oil mold. Bring cranberries, sugar, and 3 cups water to a boil in a large saucepan, then reduce heat. Cook, stirring occasionally, until cranberries burst and are completely softened, about 10 minutes. Pour through a fine sieve set over a bowl, pressing very firmly on solids to release as much juice as possible (you should have 4 cups; if not, add water to make 4 cups). Skim any foam from surface. Stir in maple syrup.
Transfer 1 cup cranberry liquid to a clean saucepan, and bring to a simmer. Meanwhile, sprinkle gelatin over 1/2 cup water in a small bowl, and let soften 1 minute. Stir gelatin mixture into hot liquid until gelatin dissolves completely. Pour cranberry-gelatin mixture into cranberry-maple mixture, and stir to combine. Place bowl in a larger bowl of ice water. Let cool, stirring occasionally, until cool to the touch and beginning to thicken, about 15 minutes. Pour mixture into mold, and refrigerate until set, at least 8 hours and up to 2 days.
To unmold, dip top of mold into a bowl of hot water 30 seconds, then run a thin-bladed knife around edge to loosen. Place a serving platter on bottom of mold, and quickly invert. Tap to release, and remove mold. (If jelly doesn't release easily, return to hot water for a few seconds.)