Over the last year I noticed several people futilely looking for a function to do this - so I when I got interested wrote one -
The Impulse Source permits one to view the frequency response of any filter over the entire frequency range of interest. It is an out of the tree module that installs easily on several systems. The you tube video demonstrates its use and has the install instructions.
It is part of the gr-extras package that can be found at -
The simple filters do what you expect, but throw in a Decimation, Interpolation or Frequency Xlating and I found soom rather unexpected results.