Combine the yeast, honey, and warm milk (around 100°F) in a medium-sized bowl. Stir to combine and let it sit for 10 minutes until the mixture puffs up quite a bit. Add the warm water, chia seeds, oil, and vinegar and whisk together until combined. Let this mixture sit for another 2 minutes to allow the chia seeds to expand.
While the chia seeds are expanding, mix together your dry ingredients (the GF flour, buckwheat flour, baking powder, baking soda, salt, and xantham gum) in a large bowl. Add the wet mixture to the dry ingredients and stir until just combined. Pour the mixture into a 9ʺ x 5ʺ greased pan and press the dough down with the back of your spoon to flatten out.
Place the pan in a warm (but non-drafty) spot to rise for 30-45 minutes until the dough rises past the top of the pan. Since it's not very warm outside yet, I turned the oven on Preheat for just a minute or two and then turned it off and placed the bread inside to rise. This worked really well for me, but if you find your bread rises too much and overflows the pan, try placing it in the oven without preheating.
Once your bread is done rising, preheat your oven to 350°F and bake the bread for 60-70 minutes until the top is golden brown. Let the bread sit in the pan for 5 minutes and then transfer it to a cooling rack to completely cool before slicing.