There is a lot to be said about randomness, but for the purpose of
getrandom() and getentropy(), which to my knowledge aren't globally
standardized (de-factor or otherwise) and well-specified what semantics
re cryptographic properties they should have, I agree it make sense to
keep the implementation small and using gc-random probably causes more
problems than it solves.

Historically, the problem is that for cryptographic purposes,
/dev/random and /dev/urandom can be a really bad choice on many
platforms.  This has probably been improved over the years, especially
on the most relevant platforms, but still: for any application that
needs portable randomness for some particular crypto property, the
getrandom, getentropy, and /dev/*random interfaces are not sufficient. 
The gc-random module wasn't really perfect in this regard, it required
that people used libgcrypt or provided a known-good randomness device
that is different for every platform .  The gc-random logic was
incomplete here.

There ought to be a word about in the gnulib documentation for
getrandom() and getentropy() so that applications don't assume these
gnulib modules provides crypto-strength output on all platforms.

/Simon