To add a certificate (for example your or your organization's internal CA certificate) to the systemwide list of trusted certificates, place the new certificate in /usr/local/share/ca-certificates/. Be sure, that certificate name ends with .crt (and not with .pem).
mkdir -p /usr/local/share/ca-certificates/
cp path/to/my.crt /usr/local/share/ca-certificates/
Make sure users have necessary read access to the directory/certificates, or users won't trust them due to read failures. Also make sure untrusted users don't have write access!
Then either run
or rebuild app-misc/ca-certificates with
emerge -1 app-misc/ca-certificates
to add the certificates to /etc/ssl/certs/.
Chromium (like many other web browsers) do not use the systemwide CA certificate list.
You need to open chrome://settings/certificates then choose Authorities tab and import your CA certificate.