This is a sample project which shows how PayPal JS SDK (https://developer.paypal.com/docs/checkout/integrate/) can be integrated into a GWT app.
It also serves as an example for JSInterop.
Thank you Ahmad K. Bawaneh and Colin Alworth for your help (or better: for all the hard/main work). My contribution to the project is very small.
In src/main/webapp/PaypalWithGTW.html find
<script src="https://www.paypal.com/sdk/js?client-id=SB_CLIENT_ID¤cy=EUR"></script>
and replace SB_CLIENT_ID with your own cliend-id (any maybe change the currency setting).
Get your client id from the Paypal Developer portal. Follow the guide here: https://developer.paypal.com/docs/checkout/integrate/
Create a sandbox buyer account, so you can "buy" stuff in the Paypal sandbox environment by paying with "virtual money".
Start the application with:
mvn gwt:devmode