After installing GnuPG from Homebrew, gpg was unable to connect to one of its key servers:

$ gpg --refresh-keys
gpg: refreshing 47 keys from hkps://hkps.pool.sks-keyservers.net
gpgkeys: HTTP fetch error 60: SSL certificate problem: Invalid certificate chain
[...]

The trick was to install their root certificate and mark it "trusted":

$ wget https://sks-keyservers.net/sks-keyservers.netCA.pem
$ open sks-keyservers.netCA.pem
	=> Trust always

Now the operation was able to complete:

$ gpg --refresh-keys
[...]
gpg: Total number processed: 47
gpg:              unchanged: 19
gpg:           new user IDs: 5
gpg:            new subkeys: 4
gpg:         new signatures: 1698
gpg:     signatures cleaned: 2
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:  19  signed:  12  trust: 0-, 0q, 0n, 0m, 0f, 19u
gpg: depth: 1  valid:  12  signed:   4  trust: 12-, 0q, 0n, 0m, 0f, 0u
gpg: next trustdb check due at 2018-08-19