Storage:
Gravy gets thicker and gets a skin on the surface as it cools.
Just use water to thin out, whisk the skin in and heat it up, it will be as good as new.
If serving soon, just place a lid on the saucepan then reheat when required.
Gluten-Free Option:
Skip flour, ensure stock cubes are gluten-free.
Mix 2.5 tbsp cornflour/cornstarch with 3 tbsp water, then add that into the hot stock liquid.
Mix, then pour that into the saucepan with melted butter.
Will thicken in 2 minutes or so.