gnutls-trustfiles

gnutls-trustfiles is a variable defined in `gnutls.el'.
Its value is

("/etc/ssl/certs/ca-certificates.crt" "/etc/pki/tls/certs/ca-bundle.crt" "/etc/ssl/ca-bundle.pem" "/usr/ssl/certs/ca-bundle.crt")

Documentation:
List of CA bundle location filenames or a function returning said list.
The files may be in PEM or DER format, as per the GnuTLS documentation.
The files may not exist, in which case they will be ignored.

You can customize this variable.