Preheat oven to 350 degrees Fahrenheit. Line bottom of 9in round pan with parchment paper and grease sides of pan.
Combine 1/4 cup vegan butter and 3/4 brown sugar in a pan and bring to a boil over medium heat while stirring constantly. Pour butter mixture into greased and lined 9in round pan.
Cut lemons thinly and de-seed carefully with knife. Start by placing a small lemon slice in the middle of the pan on top of the butter mixture and create rings of lemon around center slice. Slices should overlap a bit. Ensure the whole bottom of the pan in lined with lemons.
In a bowl, whisk the GF flour, sugar, salt, baking soda and powder. In another bowl, whisk the olive oil, almond milk, flax eggs, lemon zest and juice and Grand Marnier. Add the dry ingredients; whisk until just combined.
Pour the batter into the prepared pan on top of lemon slices and bake for 1 hour and 20 minutes, until the top is golden and a toothpick comes out clean. Transfer the cake to a rack and let cool for 30 minutes.
Place a large pan underneath rack to catch any butter drippings. Use a large plate to carefully flip cake and then place on rack. Be careful, butter mixture will be runny!
Let cake cool and firm up for 2 hour. Slice and enjoy!