How to insert into view through GUI in pgAdmin?
Sorry, no luck with pgAdmin. Per documentation:


  Note that views can’t be edited and are read only by design; updatable
  views (using rules) are not supported at this time.


There is an item in the project's TODO list:


  
  Updateable View wizard
  


Seems like nobody tackled that, yet. (Contributors welcome.)
There have been requests to introduce this feature in the past. You could email pgadmin-support@postgresql.org to document popular demand ...

For now, it seems like Navicat supports it.