Preheat oven to 350F (180C). Grease and flour (with some gf all-purpose flour blend) a large (e.g., 10-cup or larger) Bundt pan.
In a small bowl, combine the water and flaxseed meal. Let stand 5 minutes.
In a large bowl, whisk the gluten-free flour, sugar, baking powder and salt until blended. Add the flaxseed mixture, milk, oil and vanilla; whisk or stir until completely blended and smooth.
Pour and spread the batter evenly in the prepared Bundt pan.
Bake in the preheated oven for 55 to 60 minutes until a long skewer (an uncooked piece of spaghetti is perfect) inserted in the center comes out with only a few moist crumbs attached.
Place the pan on a cooling rack and cool in the pan for 10 minutes. Use a butter knife, if needed, to loosen cake at the sides. Place a cooling rack on top of the pan and invert the cake. Cool completely.
Dust with confectioners' sugar, if desired.
Notes
Storage: Store the completely cooled cake in a cake container or other covered container at cool room temperature for up to 3 days. Store in an airtight container in the refrigerator for up to 1 week. The cake can also be stored (whole, or sliced) in an airtight container in the freezer for up to 3 months.Tip: If you do not need this cake to be Gluten-Free, replace the gluten-free flour blend with 2 and 1/2 cups all-purpose flour + 1/4 cup cornstarch, arrowroot, or potato starch.Tip: Make sure that the all-purpose gluten-free flour blend that you choose is a 1:1 replacement (i.e., it can be used measure for measure for all-purpose flour) and that it contains xanthan gum.