Combine flour, water, eggs, egg yolk, vegetable oil, sugar, yeast, and salt in the bowl of a stand mixer. Mix with the flat beater attachment until a sticky, dough-like batter forms, about 1 minute on medium speed. Cover dough with plastic wrap and set aside for 20 minutes, until the dough is noticeably firmer and less sticky
Transfer dough to GF floured surface and gently knead, folding in half towards you then patting down, turning 90 degrees, and folding it again. Continue until dough is firmer to the touch, smooth, and easier to handle, about 15 folds. Sprinkle with GF flour as needed to prevent sticking. Transfer dough to a bowl, cover, and allow to rise for 40 minutes until puffy and increased in size by about 1.5 times
Line a baking sheet with parchment paper
On a GF floured surface, gently deflate the dough and repeat the kneading motion about 6 more times to bring the dough together in a smooth, cohesive ball.
Divide the dough into three equal pieces (about 270 g each) and roll into three logs. Braid loosely, pinching at the ends
Transfer challah to prepared baking sheet and brush dough with egg wash. Cover and allow to rise for 15 - 30 minutes, until just puffy (underrising is better than overrising). Preheat oven to 350F during this rising step
Brush the braid with egg wash again and sprinkle with seeds as desired
Bake 30-40 minutes, until well browned. A digital thermometer will read between 190F and 205F when done
Remove from oven and allow to cool completely before slicing