Digital Certs for GVC vpn

Hi, I've been looking through endless articles for a step by step guide to moving from preshared keys to digital certs on the TZ and on the GVC clients

I have a group of users who use GVC to access company resources.

I'm still not confident how it will work though.

At the moment when I select "IKE using 3rd Party Certs" there is no valid certs available.

So I figure that I need a third party cert from godaddy or similar

Do I just buy an SSL cert ?

Do I need to tell godaddy the IP address of the firewall? because the yourdomain.com points elsewhere.

Is there a server cert and a client cert?

Any direction is appreciated

Thanks