I have the very same problem.
We are hosting a Joomla Installation 2.5.28 of our customer which will not migrate to Joomla 3.X (I know, it's a bad decision but out of our influence)
But when I try to install the Gantry 4.1.33 Update I'm always getting the same error message.
Fatal error: Call to undefined method JInstallerPackage::getManifest() in /home/USER/public_html/tmp/install_59ca3fa678b08/install.php on line 75
Regarding your prior posts in this thread I uninstalled all unused templates and search for rokupdater - which is not installed.
But I'm still not able to update the Gantry Framework.
Can you please help me with this?
Thanks in advance.
With kind regards