It depends on two things.
First, make sure that the "System - Admin Tools" plugin is the first system plugin to fire. You may have to change its order to -100000.
Then, we have the specific PHP version. Joomla! is hard-coded to output its own HTTP header when GZip is enabled, no matter what. Admin Tools will also output its own header, instructing PHP that it should replace any existing headers. Supposedly, PHP should silently replace the header and be done with it. Not all PHP versions on all platforms do.
Nicholas K. Dionysopoulos
Lead Developer and Director
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