Acidify your solution, GOx will flocculate at low pHs. You can test it by adding HCl little by little and then centrifuge.

I think the most efficient and simple way is to use filtration.

I agree with Christian Punckt and Lisandro Cunci ·.Adding salt, like NaCl, or HCl is an effective way to flocculate GO. After that, you can use dialysis method to remove NaCl to get the water soluble GO back.

Thank you so much for your answer.

I used different salts such as NaCl and KI. I have another question. if we diapers GO in ethanol we can separate GO from solution easily but by using water we need to use salt. why?

Following the answer from Christian Punckt:

'suspensions of GO are stabilized through the charges present on the GO sheets as a result of the dissociation of acidic surface functional groups'

This process occurs badly in ethanol (check the concepts of weak acid dissociation in ethanol in any Chemistry textbook), therefore dispersions are not as stable and then you can separate GO very easily.