I take your point that things could be more clearly spelt out on the GPC web site about the current state of play.
Absolutely.
But we did discuss this matter at length in 2010 - see http://www.g-n-u.de/pipermail/gpc/2010-July/thread.html
When I started trying to use GPC in March, 2016 the whole GPC website was defunc, but I was able to dig out the contact person (Peter Gerwisnki) and he resurrected the pages. I also had a short PM exchange with him about the slow death of GPC, and he pointed me to the 2010 thread and specially Frank Heckenbach's summary, which I successfully found at archive.org: http://web.archive.org/web/20140714170318/http://fjf.gnu.de/gpc-future.html
I do not know, if more groups have problems with keeping their projects up with gcc development, but I know that GNU Modula-2 has similar problems.
With some work, I was able to build a working GPC on TinyCore Linux, but apparently I've lost my notes how I did it. I remember that I had to combine several patches & tips from the mailing list, but I had success.
I agree with Ken that it is not no straightforward way to get a running gpc on a standard Linux. TinyCore is quite special in having very few tools only in the default setup and you generally can use older packages without big problems, but the more mainline distributions have interdependencies, which make it quite difficult to downgrade to older versions.
-- Bernhard