Yes driving uphill will use slightly more gas than driving downhill or on flat ground.
When you're going uphill in your vehicle your vehicle requires more engine power so you'll be pressing the gas pedal down a bit further so that your engine can get more gas to provide more engine power.
So it's normal for your vehicle to use a bit more gas as when the gas pedal is pressed down further the throttle is open wider and more gas is introduced into the engine.
If you do a lot of uphill driving then your vehicle will consume more gas than it would if you were just driving on flat land.
But if you're only driving uphill a short amount and not doing it often you should not have much of a decrease in fuel mileage.