Generally it's whatever you've specified in the
keyserver option (either in command line, or in
~/.gnupg/gpg.conf, or recently even in
The default may vary depending on version and distribution. However, in most cases it's a large pool of servers (usually the SKS pool), where the actual server addresses may change frequently. If you are behind a firewall which requires whitelisting, you should always configure the server manually.
Latest GnuPG 2.2.x actually use the SKS "hkps" pool, which only needs port 443 (HTTPS) and should work with most firewalls. (It does however require DNS SRV record support.)
gpg --keyserver hkps://hkps.pool.sks-keyservers.net:443 ...
Note that SKS even provides a "p80" pool, which speaks HKP entirely over port 80 (the standard HTTP port) and should work with any firewall or proxy server. For example:
gpg --keyserver hkp://p80.pool.sks-keyservers.net:80 ...