A gas giant is one of the types of planets in the game. Gas Giants cannot be landed upon, as they have no surface. You will, however, float just above the cloud layer.
Gas Giants provide resources in the form of gas, and many different types of gases are available. When viewed from the solar system, the Gas Giant's details will show the gases it produces. Do not be alarmed by the very low rate show - this rate is per Orbital Collector installed, so you can increase the rate by placing more Orbital Collectors.
Orbital Collectors may only be placed on the Equator of the Gas giant (where the North/South Latitude = 0. It is marked by a thicker green line in construction mode), so the maximum limit of throughput of gas from a Gas Giant is dependent upon how many orbit collectors can be placed around the equator.
Note that when Orbital Collectors are in use, they power themselves by burning off some of the gas they draw in, so your throughput will differ from what the solar system screen shows.