If there are that many Jupiter+ sized objects then how many rocky planet sized ones are there? Would they even be detectable via infrared or gravitational lensing?
Rocky planets can only form in proto disks around newly forming stars. Heavier materials (carbon and up) are concentrated this way.
If there are a lot of massive objects out there (rogue planets, maybe primordial black holes) then these could be used to aid acceleration and deceleration through gravity assist and the Oberth effect. (The latter describes how you get more acceleration thrusting near a massive body.)
You could fly along with an ultra sensitive gravity telescope / sensor and hop between massive objects.
You could also perhaps harvest propellant from some of these.
Long duration interstellar flight would be easier if the space between solar systems was not as empty as we think.
edit: I did the most basic possible amount of research by reading the first paragraph of the wikipedia article (can we call that a milliHbomberguy?), and you're right, you do gain more velocity than just firing your engine in space. Although it puts a limit on where you'll be going after your approach.
An Oberth effect flyby of a primordial black hole with a high impulse drive like nuclear pulse propulsion would be pretty spicy. It’s a feasible way to achieve speeds close to the speed of light. Acceleration would kill you though.
If we did find such an object anywhere within a few light years of the sun it would be a way to send probes out at significant fractions of the speed of light.