Using NEW on tables with generated columns
That is simple: use an AFTER trigger instead. The column value will have been computed then.
On the other hand, your answers reveals that you need a BEFORE trigger because you want to modify NEW.
So probably the best solution is to use a BEFORE trigger with a WHEN condition that excludes the generated column:
WHEN ((NEW.col1, NEW.col2, ...)
      IS DISTINCT FROM
      (OLD.col1, OLD.col2, ...))