Can gravlax be prepared from frozen salmon?
Yes, you can.  If the frozen salmon is of high quality, there is no reason based on the food science that it should not work.  

Many of the recipes and articles on the web indicate that you should (or sometimes can) start with frozen salmon.  Some, including Cooking for Engineers, specifically recommend using frozen salmon to reduce the risk from parasites:


  Parasites, such as roundworms, can naturally reside in salmon
  regardless of if it was frozen or fresh. For safety, use salmon that
  has been commercially frozen or freeze the salmon yourself to at least
  -10°F (-23°C) for at least 7 days.