You’ve got your gears backwards. Lower number means easier pedaling and slower speeds. So starting from stop, or going up a steeper hill would be first gear and shifting up as you get going… cruising along at 12 mph would probably be fifth or sixth. You get the idea!
The assist levels amplify the power of your pedaling, so to speak, but you still want to be in an appropriate gear. It will all come pretty quickly once you get used to it a little. I’d suggest finding a friendly cyclist to ride around with for an afternoon just to get you pointed in the right direction.
This is vastly oversimplified, but to answer your specific question, put it in a low gear and start up the hill. If you are spinning the pedals too fast, shift up a gear at a time until you find the range that is comfortable. Since you know you’re going up a hill, make sure the assist is at least in Normal or whatever the midrange Giant setting is called on that bike.
Oh, and have a blast with it! That’s a nice bike. Giants are quality machines and the Yamaha motors they generally use are top notch.