How to cite information obtained from Google Maps Area Calculator Tool?
There are a number of issues.  First, you need to comply with the google terms of Use agreement.  For maps, information is available at https://www.google.com/permissions/geoguidelines/attr-guide.html  

Next, you want your readers to be able to find the maps you're referring to.  Use the "share or embed" option on the maps page to generate a permalink.

Lastly, what should the reference look like?  APA provides some guidance at http://blog.apastyle.org/apastyle/2015/08/how-to-cite-online-maps.html

For one example they show 


  Google. (n.d.). [Google Maps directions for driving from Ingolstadt,
  Germany, to Geneva, Switzerland]. Retrieved August 4, 2015, from [url
  here]


I would change the author as specified in Google's attribution guidelines above, and I would replace "driving directions for ...." to "Area Calculator" or some such.

After doing this, I'd let the editorial process take over, having done your due diligence.