Looking for a similar solution I just came across this brand new tutorial, which lets you bridge a Swift gradient background (https://github.com/soffes/GradientView) library while walking through every step to get a working React component.
It is a step-by-step tutorial, allowing you to build your own component by bridging the swift and objective-c component into a usable React Native component, which overrides the standard View component and allows you to define a gradient like the following:
<LinearGradient
style={styles.gradient}
locations={[0, 1.0]}
colors={['#5ED2A0', '#339CB1']}
/>
You can find the tutorial here: http://browniefed.com/blog/2015/11/28/react-native-how-to-bridge-a-swift-view/