When the hkp port blocked, so rvm suggested gpg --keyserver hkp://keys.gnupg.net --recv-keys D39DC0E3 always failed to running, here is how to resolve such problem.

1. Find a hkp port not block server, run export after import the key:

   gpg --keyserver hkp://keys.gnupg.net --recv-keys D39DC0E3 gpg --export --armor D39DC0E3

2. Go to hkp port blocked server, run:

   gpg --import -

and copy and paste the step 1 server public key content and press Ctrl+D

Another option you can try is using port 80, but it’s can not always work.

apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 561F9B9CAC40B2F7