"But in Star Trek, the stuff could be hauled in gravitically, using a tractor beam."
Yes, and I believe the "ramjet" intake rings behind the nacelle caps use this kind of technology (maybe the three thingies at the underside of the nacelles' front?).
What I have difficulties imagining is the rotating mechanism behind the cap creating suction to attract / intake interstellar matter as fuel - because it would be blocked by the forward deflector shield (maybe a problem they overcame by the time of TNG?).
The interstellar matter has to "go around" the forward shield until the intake or tractor mechanism can "grab" and suck it in.
Alternately the particle flow around the deflector shield could be manipulated in such a way that the interstellar matter is simply channeled into the intakes. Kimble's TMP Enterprise
blueprints refer to these intakes as "sinks" which sounds rather passive to me.
If the built-up of interstellar matters in front of the shield occurs automatically as a side effect of warping space, a (smaller) tractor system appears to be sufficient to do the job so that these big, ominous caps can be assigned to better use.