Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from General Santos to Qatar as you normally would.
For a flight from General Santos to Qatar, you will be flying from General Santos Intl. General Santos Intl (GES) is just 7.0 km from the center of the city.
This is defined as General Santos Intl Airport (GES) and Doha Hamad Intl Airport (DOH). Codes like Doha Hamad Intl Airport are often used to just call airports uniquely.