Sorry for any confusion - there’s a coupler as @jacobturner describes that takes the air out of the GF and into the filter.
Particulate distribution across the media’s always a challenge. Fortunately we’ve got an outstanding engineer on it. We’re not just plopping a vacuum filter and a sack of charcoal briquettes in there. 
@rpegg, if I said anything to the contrary that you happen to recall, please do let me know and I’ll see about correcting or clarifying.