1. First you need to save the merged google document URL to a field on your object first (see this article for how to create Google Doc URL Field).
  2. Then you need to create a visual force page and an apex controller extension. The page must use the object controller you want to embed on. The visual force page code should include the iframe code like: <apex:iframe src=”http://www.salesforce.com” scrolling=”true” id=”theIframe”/>.  Salesforce includes standard controllers for all standard and custom objects you can use.
  3. And you would replace the URL there with the field that contains the full URL of the google document that is on the object. You can also add to the iframe code to specify more things like width and height. Read more here: https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_compref_iframe.htm.
  4. So you can simply create a Visual Force Page under setup with some code like this for example for the standard Work Order object:
    <apex:page standardController="WorkOrder">
      <apex:iframe src="{!workorder.Google_Doc_URL__c}" scrolling="true" id="theIframe"/>
  5.  Then you edit the standard page layout, create a new section on your layout with one column and click on visual force pages available and drag the one you created into the page layout and specify how high you want it.