Gas planets and terrestrial planets have lots of differences. Gas planets are huge in size, while the terrestrial planets are small. Gas planets have very strong gravitational pull, while the terrestrial planets have more moderate gravitational pull. Gas planets consist mainly (not totally) of gas (hydrogen, helium, ammonia, water, methane), while terrestrial planets are mostly made out of rock, with iron core. Gas planets have atmospheres that are very dense and multilayered that consist of the aforementioned gasses, while terrestrial planets have atmospheres that they have manufactured geologically.