Not all gas giant planets are structured like the ones in our solar system. Some have been found that have metals and water vapour in their atmospheres.
Here is an except from just one paper:
Transmission spectroscopy at higher resolution has yielded information on atmospheric dynamics, as subtle Doppler shifts of absorption lines can be used to assess the velocity and direction of atmospheric flow (Flowers et al., 2019; Miller-Ricci Kempton et al., 2012; Showman et al., 2013a). This has enabled planetary-scale wind determinations (Snellen et al., 2010), differential wind measurements between leading and trailing hemispheres (e.g., Louden & Wheatley, 2015), and constraints on the planetary rotation rate (Brogi et al., 2016). Ehrenreich et al. (2020) found evidence for strong (≈5.3 km s−1) day-night winds in the ultrahot Jupiter WASP-76b, along with evidence for condensation of iron on the planetary nightside due to a hemispheric asymmetry in its absorption signal. Recently, Tabernero et al. (2020) confirmed this detection of day-night winds for WASP-76b over a broader range of atomic and molecular signatures, including ionized calcium, atomic manganese, magnesium, sodium, potassium, and lithium. These are just a handful of the broad range of atomic and ionized metallic species and molecules that have been detected in hot and ultrahot Jupiter atmospheres with high-resolution transmission spectroscopy (e.g., Birkby et al., 2013, 2017; Brogi et al., 2014; Hoeijmakers et al., 2018, 2020; Nugroho et al., 2017; Piskorz et al., 2017; Wyttenbach et al., 2017; Seidel et al., 2019, for a recent review see Birkby, 2018).
It's also possible that space travel came to the original Dweller home world as a form of alien first contact.
In the end, I suspect that, unless someone finds a note on this from Banks, this question will be closed as opinion-based.