User Dropdown List and Custom Notification Routing in Gravity Forms. See the walkthrough:

add-upload-attachments.php

<?php
/**
 * Add the file uploads to the emails
 *
 * @author Joshua David Nelson, josh@joshuadnelson.com
 *
 * @link http://www.gravityhelp.com/documentation/page/Gform_user_notification_attachments
 * @link http://www.gravityhelp.com/documentation/page/Gform_notification
 **/

add_filter( 'gform_notification_1', 'jdn_add_email_attachments', 10, 3 );
function jdn_add_email_attachments( $notification, $form, $entry ) {

    $fileupload_fields = GFCommon::get_fields_by_type($form, array("fileupload"));

    if( !is_array( $fileupload_fields ) )
        return $notification;

    $attachments = array();
    $upload_root = RGFormsModel::get_upload_root();
    foreach( $fileupload_fields as $field ){
        $url = $entry[ $field["id"] ];
        $attachment = preg_replace('|^(.*?)/gravity_forms/|', $upload_root, $url);
        if( $attachment ){
            $attachments[] = $attachment;
        }
    }

    $notification["attachments"] = $attachments;

    return $notification;
}

custom-route-notification.php

<?php
/**
 * Change the sent to email address in the notification
 *
 * @author Joshua David Nelson, josh@joshuadnelson.com
 **/
// Route to user address from drop down list, update the '1' to the ID of your form
add_filter( 'gform_notification_1', 'route_user_email_notification', 10, 3 );
function route_user_email_notification( $notification, $form , $entry ) {
    
	foreach( $form['fields'] as &$field ) {

		// Similar to above, find the right field
		if( $field['type'] != 'select' || strpos($field['cssClass'], 'user-emails') === false )
			continue;

		// Pull out the user id selected, by the field id and the $entry element
		$field_id = (string) $field['id'];
		$user_id = $entry[ $field_id ];

	}
	
	// set the email address to send the email to
	if( !empty( $user_id ) ) {
		$email_to = get_the_author_meta( 'user_email', $user_id );
	}

	if ( !empty( $email_to ) && is_email( $email_to ) ) {
		$notification[ 'to' ] = $email_to;
	}
	
	return $notification;
}

populate-user-email-drop-down.php

<?php
/**
 * Populate the drop-down menu with users
 *
 * @author Joshua David Nelson, josh@joshuadnelson.com
 **/
// Gravity Forms User Populate, update the '1' to the ID of your form
add_filter( 'gform_pre_render_1', 'populate_user_email_list' );
function populate_user_email_list( $form ){
        
    // Add filter to fields, populate the list
    foreach( $form['fields'] as &$field ) {
        
	// If the field is not a dropdown and not the specific class, move onto the next one
	// This acts as a quick means to filter arguments until we find the one we want
        if( $field['type'] !== 'select' || strpos($field['cssClass'], 'user-emails') === false )
            continue;
        
	// The first, "select" option
        $choices = array( array( 'text' => 'Select a User', 'value' => ' ' ) );
		
	// Collect user information
	// prepare arguments
	$args  = array(
		// order results by user_nicename
		'orderby' => 'user_nicename',
		// Return the fields we desire
		'fields'  => array( 'id', 'display_name', 'user_email' ),
	);
	// Create the WP_User_Query object
	$wp_user_query = new WP_User_Query( $args );
	// Get the results
	$users = $wp_user_query->get_results();
	//print_r( $users );
	
	// Check for results
        if ( !empty( $users ) ) {
		foreach ( $users as $user ){
			// Make sure the user has an email address, safeguard against users can be imported without email addresses
			// Also, make sure the user is at least able to edit posts (i.e., not a subscriber). Look at: http://codex.wordpress.org/Roles_and_Capabilities for more ideas
			if( !empty( $user->user_email ) && user_can( $user->id, 'edit_posts' ) ) {
				// add users to select options
				$choices[] = array(
					'text'  => $user->display_name,
					'value' => $user->id,
				);
			}
		}
	}
        
        $field['choices'] = $choices;
        
    }
    
    return $form;
}

Many thanks, it really helps <3