Hi there,
I set up an OSGI config file called com.adobe.granite.ui.clientlibs.impl.HtmlLibraryManagerImpl.xml with these contents and gcc seems to work o.k.
for JS with some caveats.
I still get some files failing minification because of "strict" errors which I couldn't seem to get around with configuration options in this file.
In the end, I had to fix the errors in the files to get the gcc to minimize them.
<jcr:root xmlns:sling="http://sling.apache.org/jcr/sling/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0"
jcr:primaryType="sling:OsgiConfig"
htmllibmanager.disable.replacement="[]"
htmllibmanager.path.list="[/apps,/libs,/etc]"
htmllibmanager.maxDataUriSize="0"
htmllibmanager.useFileSystemOutputCache="{Boolean}true"
htmllibmanager.fileSystemOutputCacheLocation=""
htmllibmanager.timing="{Boolean}false"
htmllibmanager.clientmanager="granite.clientlibrarymanager"
htmllibmanager.maxage="-1"
htmllibmanager.debug="{Boolean}false"
htmllibmanager.forceCQUrlInfo="{Boolean}false"
htmllibmanager.processor.css="[min:yui]"
htmllibmanager.minify="{Boolean}true"
htmllibmanager.debug.init.js="window.CQ_initial_log_level\='INFO';"
htmllibmanager.excluded.path.list="[/etc/commerce/classifications,/etc/commerce/collections,/etc/commerce/payment-methods,/etc/commerce/products,/etc/commerce/shipping-methods,/etc/reports/auditreport,/etc/reports/compreport,/etc/segmentation,/etc/taskmanagement,/etc/workflow/instances]"
htmllibmanager.defaultthemename="default"
htmllibmanager.defaultuserthemename="default"
htmllibmanager.processor.js="[min:gcc;languageIn\=ECMASCRIPT_2015]"
htmllibmanager.gzip="{Boolean}true"
htmllibmanager.longcache.patterns="[]"
htmllibmanager.longcache.format="lc-%s-lc"
/>
It might be that some of files are minifying for you and others aren't.
Also, it might be worth clearing the clientlibs caches in AEM as described here, so you get a fresh view of them.
https://docs.adobe.com/content/help/en/experience-manager-65/developing/introduction/clientlibs.html