Preheat oven to 350 degrees F.
Grease 9x5" loaf pan.
Combine sugar with grapefruit zest and mash together with a fork, to work them together. Set aside.
Combine flours, baking powder, and salt, and whisk to combine.
In separate bowl, whisk together sugar mixture, melted coconut oil, eggs, vanilla, yogurt, and grapefruit juice until well combined.
Add in the flour mixture and mix with a spatula until combined.
Add poppy seeds and mix until incorporated, trying not to overmix.
Pour batter into prepared baking pan.
Bake in preheat oven for 50-55 minutes, until golden brown around the edges and set in the middle.
Cool partially, about 20 minutes.
Once slightly cool, make the glaze by whisking together grapefruit juice and powdered sugar. Start with 2 tablespoons of grapefruit juice and add more if it seems too thick.
Drizzle over bread.
Cool completely, then cut into slices and enjoy!