Excellent job. All other sollution explained by others member is really just wasted my time. Specially after successful installation of magento 2.3.0 if any one see blank gray admin login page. You just follow the above solution. Do not need to anything else. All other explanation For example - running command as below is just wasting your time, such as
php bin/magento setup:static-content:deploy -f
php bin/magento setup:upgrade
all these are nothing to do with this problem or anything about permission.