Gravity Forms: How to add PHP function to confirmation conditional shortcode?

In Gravity Forms, I have a confirmation set up with various conditional shortcodes. Example:

[gravityforms action="conditional" merge_tag="{:3:value}" condition="contains" value="E"]*** GET A CATEGORY ***[/gravityforms] 

Additionally, I have a PHP function that gets all posts by category, and I would like to use this function inside the conditional shortcode. I’m not figuring out a way to do this, so any help would be appreciated.

My function: useful_tools_list(array( 'type' => 'documents', desc => 'true' ))

I also made it into a shortcode: [useful-tools type="documents" desc="true"]

I tried using <?php ?>, but I realized I can’t embed PHP into the editor. The conditional shortcode doesn’t support nesting other shortcodes inside of it, so that doesn’t work.

I’m wondering if there is a way to manipulate the GF conditional shortcode to allow nesting? Or another way of calling the function that I’m not aware of?