The gzip option is in the template settings, sorry I did not realize that. Now I disabled gzip and still the gallery is not showing.
I also tested with a new gallery addon, but it is not working.
Some time ago the ISP changed PHP 7 to PHP 8, but the template did not work, and I asked to revert back to PHP 7. Now the site runs on PHP 7. In this occasion, could some changes to the PHP settings have occurred that could have impacted on the gallery addon? I also fixed some Joomla code to work in PHP 8 (Deprecated function parameters etc.), but not anything in SP Page Builder code.