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.
Kepler-124b
Kepler-124b is an extrasolar planet discovered in 2014. It is located from Earth, orbiting the unclassified star Kepler-124 in the constellation Cygnus. Within The Kepler-124 system (KOI-241) there are three known planets, Kepler-124b being both the smallest and closest to its parent star.