Verify Facebook Page Role
Facebook Pages has six different roles to access settings and publish content as a page with the Pages API. Depending on your Page role you are able to execute different actions like posting or getting insights data.
However, GAIN only requires 3 permissions: CREATE_ADS, BASIC_ADMIN, and CREATE_CONTENT.
If you’re unable to see any of the pages that you manage, check with a user who is a full admin for those pages so they can set your user with Content Creator or Full Admin permission.
Check App's Permissions
On Facebook, go to Settings
Then go to Apps...
Depending on how many apps are connected, GAIN should appear at the top, if not simply click on Show All to find it.
In the pop-up that appears after clicking on the pencil icon, the app's permissions have to look as below:
Following these steps should now allow you to see and connect all Facebook Pages you manage on GAIN.
For information on all Page roles and capabilities, see Facebook Help Center, Page Roles.