Keep a reference to your history object. i.e.

import { createBrowserHistory } from 'history';

var history = createBrowserHistory();

ReactDOM.render((
    <Router history={history}>
        [...]

Then add a listener to record each pageview. (This assumes you've already set up the window.ga object in the usual manner.)

history.listen((location) => {
    window.ga('set', 'page', location.pathname + location.search);
    window.ga('send', 'pageview');
});