Immediately after logging in this error is thrown by the server, and the admin page never renders.

This means that magnolia is effectively completely unusable under Glassfish v3.

This has been a problem in previous version of glassfish, and with previous versions of Magnolia (going back to at least 3.5, probably earlier, but I can't remember), but in previous version of glassfish the problem only caused a warning to be logged. Glassfish version 3 treats this as a fatal error and aborts rendering the page.
With previous versions of Magnolia running under glassfish 2.5 (such as sites I currently have running) it's only the admin interface that causes this problem - rendering normal pages does not cause a warning to be logged.

here's the stack trace :

java.lang.IllegalStateException: PWC3990: getWriter() has already been called for this response
       at org.apache.catalina.connector.Response.getOutputStream(Response.java:676)
       at org.apache.catalina.connector.ResponseFacade.getOutputStream(ResponseFacade.java:205)
       at info.magnolia.module.cache.filter.GZipFilter.doFilter(GZipFilter.java:106)
       at info.magnolia.cms.filters.OncePerRequestAbstractMgnlFilter.doFilter(OncePerRequestAbstractMgnlFilter.java:62)
[...]