Oh, I see what's happening; I didn't look closely enough at your first
message.
You don't really want to use gr_modtool to add new components to GNU
Radio. It's meant to be used to build separate projects that will work
with and installed GNU Radio. See, gr_modtool creates a project with
cmake files that look for installed GNU Radio libs and headers,
including gruel, so it has it's own FindGruel.cmake module that GNU
Radio does not have (because Gruel is part of GNU Radio).
What you want to do is install GNU Radio first. Then use gr_modtool to
build an out-of-tree project; that is, not inside of the GNU Radio
source code. Use "gr_modtool.py newmod" and then in gr-<your new
module> use "gr_modtool.py add" to add howto_ff. You should see that
gr-<yours>/cmake/Modules has a file "FindGruel.cmake" that the
component will use to find the installed GNU Radio packages and build
against them.
Tom