I really need to start writing my XML Europe Paper this week, and I'm readying my toolset to do so. My preferred XML editor ATM is XXE, which unfortunately isn't supported out-of-the-box by the 'abundant' set of tools being provided by the conference organisation. Leaves me with the familiar problem of having a job at hand, but spending more upfront time on the infrastructure tools rather than on the job itself. One of the glaring issues with that gcapaper.dtd thing is hyperlinked browsable DTD documentation, but fortunately the Forrest DTD documentation framework comes in handy. Funny though: I have a $$ copy of Near&Far Designer which is also capable of generating DTD documentation, but I end up using my own open source tools instead.
Update: I abused Bruno's first cut at a Forrest document-v11 XXE config, translating it into one for that GCA Paper DTD. If you want, go get it. Unzip in the XXE config directory, add gcapaper.cat to the list of XML catalogs in the XXE options menu, and off you go, without expensive trial-licensed bloatware on your harddisk. It's nothing fancy and it will only help you to get started, but hey, I'd better start writing that paper now ;-)