demo.js

import React from 'react';
import ReactDOM from 'react-dom';
import {Gmaps, Marker} from 'react-gmaps';

const coords = {
  lat: 51.5258541,
  lng: -0.08040660000006028
};

const App = React.createClass({

  getInitialState() {
    return {
      lat: coords.lat,
      lng: coords.lng
    };
  },

  onDragEnd(e) {
    this.setState({
      lat: e.latLng.lat(),
      lng: e.latLng.lng()
    });
  },

  render() {
    return (
      <Gmaps
        width={400}
        height={400}
        lat={coords.lat}
        lng={coords.lng}
        zoom={12}>
        <Marker
          lat={this.state.lat}
          lng={this.state.lng}
          draggable={true}
          onDragEnd={this.onDragEnd} />
      </Gmaps>
    );
  }

});

ReactDOM.render(<App />, document.getElementById('gmaps'));