You can use React Native Linear Gradient library for achieving this.
The API is very simple, e.g.
<LinearGradient
colors={['rgba(192,32,64,0.9)', 'transparent']}
style={{position: 'absolute', left: 0, right: 0, bottom: 0, height: 200}}
start={{x:0, y:1.0}}
end={{x: 0, y: 0}}
/>
Please, see my Expo Snack example for details: https://snack.expo.io/@zvona/linear-gradient.