Link flatpak.org

It has useful information

download.phtml

@@ -38,7 +38,7 @@
   <h3><?php echo T_("Sandbox")?></h3>
   <ul class="imagelist">
     <li class="imagelistitem" style="background-image: url(<?=$top_dir?>/images/icons/flatpak.png);">
-      <p class="justify"><?php echo T_("Flatpaks are bundles, which contain beneath the program also all required libraries. If your distribution is too old, to offer all required libraries, but has the package <span class=\"bold\">Flatpak</span>, you can install a recent version of <span class=\"gnucash\">GnuCash</span> from")?></p>
+      <p class="justify"><?php echo T_("Flatpaks are bundles, which contain beneath the program also all required libraries. If your distribution is too old, to offer all required libraries, but has the package <a href="<?php echo $URL_FLATPAK;?>">Flatpak</a>, you can install a recent version of <span class=\"gnucash\">GnuCash</span> from")?></p>
       <p class="justify"><a href="<?php echo $URL_FLATHUB;?>"><img width='240' alt='Download on Flathub' src='https://flathub.org/assets/badges/flathub-badge-en.png'/></a></p>
     </li>
   </ul>

externals/global_params.php

@@ -66,5 +66,6 @@
      $download_path="https://downloads.sourceforge.net/sourceforge/gnucash/";
      // TODO: gets redirected to https://sourceforge.net/projects/gnucash/files/ replace?
      
-     $URL_FLATHUB="https://flathub.org/apps/details/org.gnucash.GnuCash"
+     $URL_FLATHUB="https://flathub.org/apps/details/org.gnucash.GnuCash";
+     $URL_FLATPAK="https://flatpak.org/";
 ?>