Activate the yeast: Warm the milk to 105 degrees F in the microwave or on the stove. Stir in the sugar until it dissolves, then add the active dry yeast and stir. Set aside for 5 minutes or until the yeast begins frothing. If there is no frothing, your yeast is bad and you need to try again with new yeast.
Whisk the flour mixture: Meanwhile, whisk together the gluten-free flour, psyllium husk powder, and salt. Lightly oil a loaf pan while you wait for the yeast mixture to froth.
Make the dough: Make a well in the center of the flour and add the milk mixture. Stir together using a wooden spoon. When stirring becomes difficult, add the oil and continue mixing. When stirring becomes difficult again, lightly flour your hands (with gluten-free flour) and continue mixing the dough until it is uniform. Then let it rest for 5 minutes while the flour and psyllium husk powder absorb the liquid.
Knead: Lightly dust a smooth surface with gluten-free flour and dust your hands with flour as well. Scoop up the dough and begin kneading, just like you would with traditional bread. You will need to knead this dough for 15-20 minutes before it passes the standard tests for properly kneaded dough. These include the poke test and the "window test". See the notes for more info on kneading if you are new to this skill. This bread will go through a phase of being very sticky, about 10 minutes in. You need to continue kneading through this sticky phase. Lightly flour your hands as needed, but don't overdo it.
Oil and form a loaf: When the dough passes the kneading tests, pour and spread just a little bit of oil (about ½ teaspoon is fine) along the bottom of the original mixing bowl. Place your dough ball in the bowl and gently turn the ball so that all sides are very lightly coated with oil. Then gently form the ball into a shape that's a cross between a log and an oval that fits neatly in your loaf pan, with any seams placed underneath the dough along the bottom of the pan.
Let rise: Loosely cover the dough with a towel and set in a warm place to rise for 2 hours or until the loaf has doubled in size. See notes for suggestions if your house is cold. As you near the end of the rising time, preheat the oven to 425 degrees F.
Bake: Bake the loaf at 425 degrees F on a rack set in the bottom ⅓ of the oven for 35 minutes. Then reduce the temperature to 375 degrees for an additional 30 minutes, or until the loaf is finished baking. You know the sandwich bread is finished baking when the crust is solidly golden brown in color, tapping a fingernail on the crust creates a hollow sound, and the internal temperature of the loaf is 205 degrees F (temperature is the most accurate test for doneness).
Cool: Allow the loaf to cool in the pan for 10 minutes. Then gently transfer to a cooling rack to cool completely before slicing and eating.