Preheat your oven to 325°F (163°C). Wrap the outside of a 9-inch springform pan with two layers of heavy-duty aluminum foil and spray the inside with non-stick cooking spray.
Pulse gluten-free graham crackers in a food processor to fine crumbs, then mix with 3 tablespoons sugar and 6 tablespoons melted butter until combined and resembling damp sand. Press firmly into the bottom and 1 inch up the sides of the prepared pan. Bake for 8-10 minutes until lightly toasted, then cool.
Beat room-temperature cream cheese on medium-low speed for 2-3 minutes until completely smooth with no lumps, scraping down sides frequently.
Add 1 cup sugar, cornstarch, and salt, beating on low until combined. Add vanilla extract and lemon juice, beating until incorporated. Scrape down bowl thoroughly.
Add sour cream and beat on low until smooth. Add eggs one at a time, beating on the lowest speed just until the yolk disappears. Do not overmix. Scrape and briefly mix by hand with a spatula.
Pour filling over the cooled crust and smooth the top. Place the springform pan inside a large roasting pan and carefully pour hot water into the roasting pan until it reaches about 1 inch up the sides of the springform pan.
Bake at 325°F (163°C) for 55-65 minutes. The cheesecake is done when the outer 2-3 inches are set but the center still jiggles slightly like Jell-O. Do not open the oven door during the first 45 minutes.
Turn off the oven and leave the cheesecake inside with the door closed for 1 hour. Then open the door slightly and cool for another 30 minutes. Remove from the water bath and run a thin knife around the edges. Cool at room temperature for 1-2 hours, then refrigerate for at least 4 hours, preferably overnight.
Remove foil from the pan, run a knife around the edge again, and release the springform ring. Transfer to a serving plate, slice with a sharp knife wiping the blade between cuts, and add toppings just before serving.