Gating function?

Other than gr_pwr_squelch_xx, there doesn’t appear to be a simple gating
function in Gnu Radio–is that true?

I tried gr_mute_ff(), but it injects zeros, rather than gating the
stream, which is precisely what I don’t want.

The gr_pwr_squelch_ff() function appears to chew up a lot of CPU,
which I can’t afford.