You can easily add an A record for the subdomain by doing the following
1.Log in to your Go Daddy account.
2.Next to Domains, click Launch.
3.Click the domain name you want to update, and, in the DNS Manager section, click Launch.
4.Click the Add New Record. The Add DNS Record window displays.
5.The Host Name will be your subdomain and the Points to IP Address will be the IP Host Gator supplied you.
7.Click Save Zone File, then click OK. The new A record displays in the A (host) section.
hope this helps