Hmm, I tried what you proposed and downloaded the file from an alternative site and then kill the stuck ftp process half hoping that it would continue and use the file I downloaded but... the build process tried all by itself to download the file using the next alternative site!
Code:
===> py-crypto-2.6.1p2 depends on: python->=2.7,<2.8 -> python-2.7.12
===> py-crypto-2.6.1p2 depends on: gmp-* - not found
===> Verifying install for gmp-* in devel/gmp
===> Checking files for gmp-5.0.2p3
>> Fetch ftp://ftp.gmplib.org/pub/gmp-5.0.2/gmp-5.0.2.tar.bz2
Killed
>> Fetch http://ftpmirror.gnu.org/gmp/gmp-5.0.2.tar.bz2
gmp-5.0.2.tar.bz2 100% |********************************| 1977 KB 00:08
>> (SHA256) gmp-5.0.2.tar.bz2: OK
I simply found the process id using
ps -A | grep ftp...
Once more, thanks for your help!