Can I use a certificate authority with Elgamal encryption? Is this type of keys in CA?
Certificate Authorities (CAs) don't do encryption. They sign Certificate Signing Requests (CSRs) and some additional data (dates, authority ID, allowed uses, etc) to produce a Certificate. That certificate is the CA's attestation that the Subject's public key in the certificate is legitimately owned by that Subject. Certificates are then used for signing things by the Subject (eg signing a TLS handshake or code signing depending on the allowed uses in the certificate).