Hi @nmanthena try this function. I am note sure if the Permission is required so you can also try without ii 
_downloadFile = async () => {
const { status } = await Permissions.askAsync(Permissions.CAMERA_ROLL);
//permission for camera_roll
if (status === "granted") {
//store the cached file
const file = await FileSystem.downloadAsync(
link,
FileSystem.documentDirectory + "filename.jpg"
);
//save the image in the galery using the link of the cached file
const assetLink = await MediaLibrary.createAssetAsync(file.uri);
console.log(file, assetLink);
}
};