Heat oven to 325°F (163°C). Wrap a 9-inch springform pan in two layers of heavy-duty foil to help keep water out. Grease the sides and bottom of the pan. Set a large roasting pan on the center rack of the oven and start a kettle of water boiling for the water bath.
In a mixing bowl, stir together the gluten-free graham cracker crumbs, granulated sugar, and cinnamon (if using). Pour in the melted butter and mix until the crumbs look like wet sand and hold together when pinched; add an extra tablespoon of butter if the mixture seems too dry. Press the crumb mixture firmly into the bottom of the prepared springform pan using a flat measuring cup. Bake for about 10 minutes, then remove and let the crust cool completely.
On low mixer speed, beat the softened cream cheese in a large bowl until completely smooth and free of lumps. Add the granulated sugar and cornstarch or sweet rice flour, mixing on low and scraping down the bowl as needed. Add the sour cream, vanilla, and lemon zest (if using), and mix just until smooth. Add the eggs one at a time, mixing gently on low after each addition and avoiding whipping in too much air, since excess air can cause cracks.
Pour the cheesecake batter over the cooled crust in the springform pan. Gently tap the pan on the counter a few times to help release any large air bubbles.
Place the foil-wrapped springform pan into the roasting pan in the oven. Carefully pour the boiling water into the roasting pan until it comes 1 to 1 1/2 inches up the sides of the springform pan. Bake for 60–75 minutes, until the edges are set and the center still jiggles slightly when gently shaken. If using a thermometer, the center should register about 150–155°F.
Turn off the oven, crack the door open slightly, and leave the cheesecake inside for 1 hour to cool gradually. Remove the springform pan from the water bath, carefully unwrap the foil, run a thin knife around the edge to loosen, and let the cheesecake cool completely on a rack.
Once completely cool, wrap the cheesecake and refrigerate for at least 4 hours, preferably overnight, until fully chilled and set. To serve, slice with a hot knife, dipping the blade in hot water and wiping it clean between cuts. Serve plain or with toppings such as fresh berries, fruit compote, or caramel drizzle.