This is an example of the gform_pre_render function - pulls from a custom table of contact

gform_pre_render example

/**
 * Creates a dropdown field based on a returned SQL query of a custom db table
 *
 * @since   0.1.0
 *
 */
add_filter('gform_pre_render_7', 'populate_contacts'); // _7 relates this to a particular field
function populate_contacts( $form ){

	// Loop through fields
    foreach($form['fields'] as &$field){

        // If the field has a population characteristic of 'contact_id'
        // There are lots of options you can choose from to limit the fiels
		if($field["inputName"] == "contact_id"){
		    
		    // Call the query functions
	        $contacts = otm_query_contacts( $_GET['company_id'] );
            
            // Set our first empty option
	        $choices = array(array('text' => 'Select a Contact', 'value' => ' '));
            
            // If we have values from the database
	        if ( !empty( $contacts ) ) :

				// Loop through contacts
				foreach( $contacts as $contact ) :
                    
                    // Add the db value to the field as a choice
					$choices[] = array('text' => $contact[fname]." ".$contact[lname], 'value' => $contact[log_id]);

				endforeach;

			endif;

            $field["choices"] = $choices;
        }

    }

    return $form;
}