If we turn it back to min:yui on the configMgr, it works again, but just minfies the ES5 functions, if we use arrow functions or let/const, it doesn't transform into ES5 (the main reason why we're changing to GCC).
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.