Gas giant
The smallest known extrasolar planet that is likely a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope.
Gliese 1132
As of June 12, 2018, there are two confirmed planets and one candidate planet orbiting GJ 1132. Gliese 1132 b is the innermost planet of the Gliese 1132 system, as well as the smallest. It is very similar in size and mass to Earth, with a radius of 1.13 and a mass of 1.66 . It is slightly denser than Earth with 30% more surface gravity, meaning it has a rocky composition. Despite its physical similarities to Earth, it is considered too hot to be habitable, getting 19 times more sunlight due to its 1.6 day orbital period. In April 2017, it was revealed that the planet has an atmosphere, potentially rich in water vapor and methane.