Send typoscript parameter to userFunc

I'm trying to send a get parameter to a userFunc in order to identify a page, but it doesn't seem to work. This is what I have:

########## CATEGORY CONTENT ##########
lib.categoryContent = COA
lib.categoryContent {
    10 < styles.content.get
    10 {
        select {
            pidInList.cObject = USER
            pidInList.cObject {
                userFunc = Vendor\Provider\UserFunc\PageIdByAlias->getPageIdByAlias
                alias = TEXT
                alias.data = GP:category
            }
        }
    }

    wrap = <categoryContent><![CDATA[|]]></categoryContent>
}

And in PHP:

/**
 * Returns page ID by alias
 *
 * @return int
 */
public function getPageIdByAlias($content, $conf)
{
    $pageId = $this->pageRepository->getPageIdByAlias($conf["alias"]);
    return $pageId;
}

I have also tried:

alias.cObject = TEXT
alias.cObject.data = GP:category

But still, I only get the string GP:category in PHP. I'm using TYPO3 7.6.11

Your TypoScript is correct. However, since the rendering is delegated to a user-function the nested TypoScript properties are not executed - this has to happen in your custom user-function. The instance of ContentObjectRenderer is automatically injected to your custom class as property PageIdByAlias::$cObj.

<?php
namespace Vendor\Provider\UserFunc;

class PageIdByAlias
{
  /**
   * @var \TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer
   */
  public $cObj;

  protected $pageRepository;

  /**
   * Returns page ID by alias
   *
   * @var string $content
   * @var array $configuration
   * @return int|string
   **/
  public function getPageIdByAlias($content, array $configuration = null)
  {
    $pageId = 0;
    // apply stdWrap() rendering for property 'alias'
    // 3rd argument defines a custom default value if property is not set
    $alias = $this->cObj->stdWrapValue('alias', $configuration, null);
    if ($alias !== null) {
      $pageId = $this->pageRepository->getPageIdByAlias($alias);
    }
    return $pageId;
  }
}