simply use e.parameter:
function doGet(e) {
var app = UiApp.createApplication();
var label = app.createLabel('Parameter value a:'+e.parameter.a+' Parameter value b:'+e.parameter.b);
app.add(label);
return app;
}
publish it,
get the URL, you should obtain something like:
https://script.google.com/macros/s/AKfycbzvmy136r2B1RcIYd7QUmv9-1BBqD9WIwpt3NX9b3Wc3yZJazOj/exec
add "?a=Something&b=somethingElse" to the URL:
https://script.google.com/macros/s/AKfycbzvmy136r2B1RcIYd7QUmv9-1BBqD9WIwpt3NX9b3Wc3yZJazOj/exec?a=Something&b=somethingElse
and reload the page, you should see:
Parameter value a:Something -- Parameter value b:somethingElse
on thw browser page.