Using {$node.object.data_map.page_body.content} gives direct output from your attribute, using {attribute_view_gui attribute=$node.object.data_map.page_body} will load template releted with page_body datatype. If you will go to design/standard/templates/content/datatype/view and preview eztext.tpl you will see code
{$attribute.content|wash( xhtml )|nl2br}
$attribute in case where you use {attribute_view_gui attribute=$node.object.data_map.page_body} holds $node.object.data_map.page_body
But you can display content of your attribute without using attribute_view_gui like $node.object.data_map.page_body.content.
Correct code for attribute_view_gui is {attribute_view_gui attribute=$node.object.data_map.page_body}
Depends on needs. Output from attribute_view_gui is template based so, you can add changes in output presentation in one place (change one template e.g eztext.tpl) and changes will affect in every place of your website.
If you need some "special" output presentation in one place of website then you can omit attribute_view_gui and use $node.object.data_map.page_body.content, so display content directly from attribute.