Are low gears are used to overcome gravity while driving uphill and downhill

When you are going uphill, force of gravity is working against us to slow us down and while going downhill gravity it will cause us to go faster. In both cases we have to use the low gears to maintain the speed, this is how counteract the effect of gravity and use of low gears while going downhill and uphill.

i think its y= -4x-14