The particles in a fusion reactor are moving at 10 km/s. They don't particularly care about gravity.

For components of the reactor, gravity tends to be helpful. It's much harder to cool superconductors or generate steam if your liquids aren't flowing along the bottom. Turbines don't work nearly as well if their inlets are sputtering like a coffeemaker.

Good question. Under the influence of a magnetic field, charged particles do all sorts of counterintuitive things due to the Lorentz force, which acts at right angles to the particle velocity and the field.

One example is a sideways precession due to the interaction between gravitational acceleration g and the Lorentz force. Fortunately this effect is basically negligible, because gravity is so much weaker than the electromagnetic forces operating on the particles.

The difficulties tend to come from the geometry of the magnetic field itself. For example, in a torus shape, there is an unavoidable drift in the vertical direction due to the combination of centrifugal force in the big circumference, and the force implied by the gradient of the magnetic field.

Space also comes with a natural vacuum, not just zero-g. That might be more interesting for a reactor since you would only have to care about shielding the magnetic coils instead of building and cooling a vacuum vessel at the same time.

Most fusion is powered by gravity, so at this point, low gravity has been nothing but an impediment.