gravityflow_post_process_workflow

Fires at the conclusion of processing any workflow step. If you want your action to only fire when the workflow is complete use  gravityflow_workflow_complete

add_action('gravityflow_post_process_workflow', 'my_workflow_post_process_function', 10, 4);
function my_workflow_post_process_function(  $form, $entry_id, $step_id, $starting_step_id ) {
    // Replace your Form ID
    if ( $form['id'] == 123 ) {  
        // do stuff
    }
}