My 1965-vintage copy of Sourcebook On The Space Sciences has this to say:
The model developed by DeMarcus in 1958 is regarded at present as the most acceptable for the interior of Jupiter. It postulates that the planet consists of 78% by weight of hydrogen molecules and the remainder helium ... calculations indicate that at a distance of about 0.8 times the planet's radius from the center, the pressure becomes so high ... that molecular hydrogen turns into a metallic form ... most of the internal volume (and mass) of Jupiter would thus appear to consist of this solid hydrogen ... it should be made clear that the model just described is used largely as a basis for calculation and is not meant to represent the exact condition of Jupiter's interior.
...
Nothing is known about the nature of the surface of Jupiter, but it is probably a strange and complex system. Although much of the interior of the planet may be solid hydrogen (and helium), the pressures in the outer parts are too low and the temperatures probably too high to permit hydrogen to solidify. Hence, there may be some liquid hydrogen (and possibly methane) near the surface of Jupiter. Between this and the atmosphere, it is expected that there will be a transition layer containing solid or liquid ammonia (or both), methane, and possibly ice. The foregoing is admittedly speculation, since the temperature distributions in the atmosphere and interior of the planet are quite unknown.
This seems to agree to some extent with the modern picture of Jupiter's structure as given by Wikipedia:
Jupiter is thought to consist of a dense core with a mixture of elements, a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen ... The core is often described as rocky, but its detailed composition is unknown, as are the properties of materials at the temperatures and pressures of those depths ... The presence of a core during at least part of Jupiter's history is suggested by models of planetary formation that require the formation of a rocky or icy core massive enough to collect its bulk of hydrogen and helium from the protosolar nebula ... A core may now be entirely absent, as gravitational measurements are not yet precise enough to rule that possibility out entirely ... The core region is surrounded by dense metallic hydrogen, which extends outward to about 78% of the radius of the planet.
That is to say, both then and now, the belief is that Jupiter and the other giant planets have dense, non-gaseous interiors.
Since the 1960s, the understanding of the behavior of hydrogen under extremely high pressure has changed, and it's believed that the bulk of Jupiter's and Saturn's volumes consist of metallic liquid hydrogen.
