The difference in the composition of the terrestrial planets and the gas giants can be explained by the different condensation in the regions near and far from the sun during the formation of the solar system.

Terrestrial rocky planets are mostly made of metals, which need higher temperatures than gases in order to condense.
According to the Solar Nebula theory, temperatures high enough to condense metals were closer to the Sun, while towards the outside of the System, temperatures where lower.
This is the reason why terrestrial rocky planets are closer to the Sun, while gaseous planets are further from the Sun.

Hence, the difference in the composition of the terrestrial planets and the gas giants can be explained by the different condensation temperature in the regions near and far from the Sun during the formation of the Solar System.