Edit: Sorry I missed the “custom notification” bit, so I guess you’re using the individual field merge tag. If so, getting a table without specific styling is expected. It’s showing better than the example above when I test it: Screenshot AMkEPEmbnW.png - Droplr
But in any case having the style it has when you use the {all_fields} merge tag is not something you can do out of the box.
You would need to add your own custom inline CSS to the notification to style the table to fit your needs. You could also use the following filter to run your own custom PHP code to alter the HTML generated by the field merge tag: https://docs.gravityforms.com/gform_merge_tag_filter/