An IAM statement which allows the Query operation on a "EventsTable" and a GSI called "eventIdIndex"

iam-ddb-query-table-and-index.js

    {
      Effect: 'Allow',
      Action: ['dynamodb:Query'],
      Resource: [
          // assuming a table "EventsTable" with a GSI called "eventIdIndex"
          {'Fn::GetAtt': ['EventsTable', 'Arn']},
          {'Fn::Join': [ '/', [{ 'Fn::GetAtt': ['EventsTable', 'Arn' ] }, 'index', 'eventIdIndex' ]]}
      ]
    }