What are the legal considerations to build a service like Drupal Gardens and earn profit from it?
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.