Turbo - when replacing timing belt, should head gaskets be replaced so as not to use old parts with new parts?

What is your question?

Radiator fluid showing on engine. Came in approximately five times, same issue, never fixed. Subaru technician replace radiator caps twice, and hoses, to no avail. Finally technician decided to perform diagnostic test and is now saying that head gaskets should be replace. I am thinking that head gaskets should have been replace at the same time timing belt was replaced. Should not put old parts in with new timing belt.

What makes this problem better or worse?

How long has your 2007 Subaru Forester had this problem?

over a month. Forester is a turbo.