Hi there - it sounds like what you're actually trying to do is update your name servers, rather than mapping a custom domain to your site.
Could you please explain your end goal in more detail? Are you moving your site from WordPress.com to Wix, for example, and you're trying to point your glutenfreegut.com domain over to the new Wix site?
In this case, you should not be trying to map domains to your site, you need to log into your domain management area instead and update the name servers so they point to Wix.
To do that, head over to Store > Domains:
https://glutenfreegutt.wordpress.com/wp-admin/paid-upgrades.php?page=domains
Look toward the bottom of the page where it says Domain Administration and select "Make changes to DNS" from the drop-down menu.
Next, follow the instructions here to log in and set your name servers to ns1.wix.com and ns2.wix.com:
http://en.support.wordpress.com/domains/change-name-servers/#changing-domain-name-servers
If you need further help, just let us know.