It seems to be expected behavior: GTT data is private to the session and postgres_fdw establish its own session where content of the table is empty. But if you insert some data in f_gtt1, then you will be able to select this data from it because of connection cache in postgres_fdw.
Thanks for the explanation.
I am able to insert and select the value from f_gtt1.
postgres=# insert into f_gtt1 values (1,'gtt_c21');