When there is need to provide restricted access to a particular page in GSite, please find below the steps.
Open the Giste in the browser and click on More actions -->Sharing & permissions
Step 1: Check if Under sharing and permission we have option to manage the page level permission is ON or OFF.In case, if it is in ON status please make it to OFF, which has to be in ON state.
Click on Enable Page-Level permissions
Step 2: Go to the top page and add the users who has to be given with restricted access.
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