Neat question! The answer is: both (sort of). The outer planets (Jupiter, Saturn, Uranus, and Neptune) and the dwarf planet Pluto (it is no longer considered a planet because of its small size) are largely composed of light elements like hydrogen, helium, oxygen, and nitrogen. Pluto does contain rock, but much of its surface is thought to be made up of frozen nitrogen (Hamblin & Christiansen, 2004). Nitrogen is also the most abundant element in Earth’s atmosphere, but it is the gas phase. Pluto is so far away from the sun that it is cold enough for nitrogen to freeze (about –210 degrees Celsius at 1 atmosphere pressure). That’s really cold! There is probably some gas atmosphere around Pluto, as the freezing points of helium and hydrogen are even lower than nitrogen.

references
Hamblin, W.K. & Christiansen, E.H. (2004). Earth’s Dynamic Systems. New Jersey: Prentice-Hall.

Pluto has a core comprised of mostly iron and rock, and it has an atmosphere of mostly nitrogen during the times it is closest to the sun.

Probably neither - Pluto is probably made mostly of ice. We've never sent a probe to actually look at Pluto up close, though, so we don't know for sure. It most likely is made of the same stuff as Neptune's moon Triton, which is mainly ice.

Pluto is ice plus rock. It has a very, very thin atmosphere.

Hello and thank you for your question! Due to the small size of Pluto, scientists have needed extremely strong telescopes to study and guess what Pluto is made out of, we haven't been able to visit Pluto yet. It is thought to be made up of mostly water ice and also contain a small rocky core which contains some metals, so to answer your question it is both! The ice covering the surface of Pluto is made of a mixture of frozen nitrogen, methane, and carbon monoxide. Pluto's atmosphere is approximately 90% nitrogen, and other complex molecules. The interior is mainly made up of water ice of different phases. And its core is to have some type of rocky elements with some metal.

Pluto is a rock. The mass of Pluto is so small, that if it was a gas it wouldn't be dense enough to hold itself together. Unlike the big gas giants, that are super massive, they exert enough gravitional pull to hold itself together. On the other hand, Pluto is such a lightweight that any gas it might once have had drifted off into space long ago.