Preheat oven to 350 degrees F (180 degrees C). Line 2 baking sheets with parchment paper.
Add melted butter, brown sugar, and caster sugar to a mixing bowl. Beat with a wooden spoon to combine.
Add pumpkin puree, flax egg, and vanilla extract. Mix well.
In another bowl, whisk together flour, salt, xanthan gum, baking soda, cinnamon, nutmeg, and ginger.
Sieve flour mixture into wet ingredients, and mix everything together.
Chill the dough in the refrigerator for 1 hour to make it easier to handle. You can also freeze the dough for 20 to 30 minutes.
Roll the dough into balls. Place them on lined baking sheets.
Bake for 14 to 16 minutes, until golden brown on top. Let cookies rest on baking sheets for 5 to 10 minutes before moving them to wire racks to cool completely.