Place gluten-free flour and salt in a large mixing bowl. Whisk to combine.
Add the coconut oil and vegan butter, and use a fork (or pastry cutter) to cut together, until the mixture resembles wet sand.
Sprinkle with ice cold water, 1 tablespoon at a time, and gently mix. Keep sprinkling water as needed until it forms a dough.
Use your hands to form the dough into a disc, then place on a piece of parchment paper, sprinkled with flour.
Lightly flour the top of the dough and a rolling pin, and roll into a rectangle, about 1/8-inch thick. Flour as needed to prevent sticking.
Spread apricot preserves over crust, leaving a 1 ½-inch border.
Slide the crust (still on the parchment paper) onto an baking sheet, and refrigerate 20 minutes.
Preheat oven to 450º F.
Core and cut the apples into ⅛-inch thick slices, keeping the pieces together as much as possible. Drizzle the apples with the lemon juice.
Once the crust has chilled, place five slices on top of the apricot preserves lengthwise, gently pressing them into the dough. Place another five slices next to the first five crosswise. Continue making a crosshatch pattern with the remaining apples. Use the parchment to help fold the dough edges over the apples to make a crust, pinching the corners together to seal. Brush the apples all over with melted coconut oil and sprinkle with cinnamon. Drizzle with the maple syrup. Brush the edges of the dough with the beaten egg (or coconut oil).
Bake the galette 40-50 minutes, until crust is golden brown. Remove from oven and drizzle with additional maple syrup, if desired, and sprinkle with flaky sea salt. Slice into squares and serve warm with a dollop of cream or vanilla ice cream.