If you are on gralise 1800 mg and go back to 1500 mg what will happen is it ok to do that for headache and restless legs?

No magic here: You can certainly try to taper onto a lower dose, and if headaches or RLS kick up, you can return to higher levels. Talk to your doctor prior to titrating, as you need to closely work together.

Why?: Why do you want to titrate down? Adverse effects? You should take the lowest effective dose. Discuss your desire to reduce the dose with your doctor.