Preheat oven to 350°F. Lightly spray a 9x13 pan, set aside.
In a medium bowl, whisk together the gluten free flour, baking powder, baking soda and salt together. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on high until light and fluffy (about 5 minutes). Scrape edges and bottom of bowl as needed.
Add egg whites and beat on high speed another 2 minutes until well blended.
Add yogurt (or sour cream), strawberry pureé and vanilla. Mix until well combined.
Slowly add flour mixture on low speed and mix until just combined.
Gently pour in the milk with the mixer on low, stop when just combined, do not overmix.
Pour batter evenly into 9x13 pan and bake for about 45-50 minutes, until edges are set and center springs back when touched. You can also test the cake for doneness by inserting a toothpick in the center. If it comes out clean, it's done!