I got the following error:

cventin:~/software> gpg --verify gmp-6.1.0.tar.xz.sig
gpg: assuming signed data in `gmp-6.1.0.tar.xz'
gpg: Signature made 2015-11-01T17:04:59 CET using DSA key ID 501441DF
gpg: requesting key 501441DF from hkp server keys.gnupg.net
gpgkeys: key DDEF6956501441DF not found on keyserver
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0
gpg: keyserver communications error: key not found
gpg: keyserver communications error: bad public key
gpg: Can't check signature: public key not found
zsh: exit 2     gpg --verify gmp-6.1.0.tar.xz.sig

Then I tried on another machine, where I got:

gpg: requesting key 501441DF from hkp server keys.gnupg.net
gpg: key C1A000B0: public key "Marco Bodrato <bodrato@gulp.linux.it>" imported

showing that the key was really on the server.

I tried again on the first machine: same error.

After several dozens of minutes, I tried again on the first machine,
and the key could be imported.

In case there was some other error, this could be like the following
bug: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=642512
(note that I don't use a proxy).