Programmatically submitting a form?

I'd like to be able to programmatically submit a form, entirely within a function of mine. I'm doing this to take advantage of the PayPal addon. I've been looking through the docs but haven't figured out how to do this.

So, is there a way to do something like this? Here's some pseudo-code showing what I'd like to do:

function my_plugin_do_payment() {
    $form = new GravityFormForm($form_id);  // get some sort of form instance
    $form->setField(1, '69.95');  // then set some product/price fields, as needed for PayPal
    $form->submit();  // submit the form, sending my user to PayPal to pay for his purchase, and firing the normal gform hooks.
}

Thanks,

John