How to add a form via a custom php wordpress hook?

There are instructions on how to display a form using a PHP function call in the Documentation area of the support site here:

http://www.gravityhelp.com/documentation/page/Embedding_A_Form

To display a form using PHP you use a function call. When displaying a form via PHP you also need to use the enqueue function to make sure that the necessary scripts and styles for that form get enqueued as the form function itself does not enqueue the scripts and styles as it can be placed anywhere and WordPress only allows functions to enqueue from certain points in the WordPress page execution lifecycle.

That is explained in the documentation above along with additional documentation for the gravity_form_enqueue_scripts function call. If Thesis custom hooks are executed at a point where things can be enqueued, then you should be able to pair both functions in one location when using the Thesis hook.

You'll have to discuss proper implementation using the Thesis hook with the Thesis support team. But the function calls in the documentation above is what you will use to display the form.