exclude categories from category post-field and include all new categories

You might have to use dynamic population to exclude the old categories which you do not want shown in the drop down. Here is an explanation of how to dynamically populate a drop down with the gform_pre_render filter.

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

In the code, http://pastie.org/2105544 on line 14, you will be retrieving categories, not posts, and you will be excluding some categories. That line, for your application, might look like this:

[php]
$args = array(
	'type'                     => 'post',
	'orderby'                  => 'name',
	'order'                    => 'ASC',
	// show only children of parent category 3
	'parent'                    => '3',
	// exclude categories 4, 7, 12 and 25
	'exclude'                  => '4,7,12,25',
	'taxonomy'                 => 'category');
$categories = get_categories($args);

Then, loop through the categories, not the posts.

Here is the documentation for get_categories:
http://codex.wordpress.org/Function_Reference/get_categories

Please let us know if you need more help.