Unfortunately, no, you will not be able to edit the robots.txt directly on GoCentral. It is a proprietary GoDaddy product and all server side access is controlled by GoDaddy. The customer has no direct access.
If that's something you need, I'd suggest using a cPanel or Plesk hosting account and build your site on a CMS of some sort.
There are also several ways to increase you SEO on the GoCentral builder within the program itself.
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