You need to go to your domain host. And add Gigarocket's name servers to your domain at your domain host first.
ns1.gigasite.co.uk
ns2.gigasite.co.uk
Then give it some time to propagate.
You then need to add the domain with the add on tool in your cpanel. Once you've added the domain all of the records will be automatically generated by cpanel. No need to edit them.