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.
K2-138b
K2-138b is the smallest planet of K2-138 with a radius of 1.57 , meaning it could be rocky. It orbits its host star every 2.35 days at a distance of 0.0338 AU. At this proximity, the planet is likely very hot and receives 486 times the stellar flux as Earth.