You are confusing the "mass" with "solid". All matter has mass, and all mass produces a gravitational field. That includes gasses, liquids and plasmas.
Although gasses are much less dense than solids, gasses also have mass, and if you have enough gas it will have a measurable gravitational field.
Jupiter is big, it is composed of lots of Hydrogen and Helium (and some other gasses), and deep within the planet, the gasses are compressed into strange states. There may even be a rocky core, but it is under such extreme pressure that it is not much like "rock" as we understand it. But it is not necessary for a planet to have a solid core to produce a gravitational field, because all matter has mass not just solid matter.
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