I understand your need now. Since GTG 0.2.9, there is a search feature.
You can search through tasks using a simple query language. You can
easily facilitate it for you:
!not @p --> shows all tasks which doesn't have @p
!not @p !not @np --> shows all tasks without tags @p, @np
@p !or @np --> shows all tasks with a tag @p or @np
You can write the query into quick add toolbar and from the completion
menu choose Search. If your query is malformed, the search option is
hidden.
After every search, the query is saved as a special tag. You can rename
it to your needs.