The -setdefault option is deprecated and no longer has any impact. Instead, you are expected to explicitely declare the certificate that you want to use for SSL.
This assumes you have reviewed IBM GSKit (Global Security Kit) - Getting Started. If you did not use the "-default_cert yes" option when adding a certificate, you can set a certificate as the default certficate using the "-setdefault" option.
ihs_home/gsk8/bin/gsk8capicmd_64 -cert -setdefault -db /path/to/example.kdb -stashed or -pw your_password -label "certificate_name"
You can list the certificates in the key database to determine which certificate is the default certificate. In this example, the certificate named "example.com" is the default certificate.
Certificates found * default, - personal, ! trusted, # secret key * example.com - bogus.com