What are the legal considerations to build a service like Drupal Gardens and earn profit from it? I ask for your help to clarify a doubt I had on the usage and licensing of the Drupal platform to build a commercial project like Drupal Gardens. 

Today, I have read both licensing and usage rules from the drupal.com and drupal.org, but I have found nothing that could give an straight answer to my curiosity about what will take both legally and economic to maintain a site 'like' Drupal Gardens. Being specific a service where people will pay to get a deployed Drupal instance.
The primary complication is the use of the Trademark "Drupal" in the product or service's name or domain. You'll need to contact the Drupal Association at http://association.drupal.org, or Dries Buytaert, the trademark holder.

If you're not interested in using the Drupal trademark as part of your product or service's name there shouldn't be any problem.