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.
HD 49674
At the time of discovery of the planet HD 49674 b in 2002, it was the least massive planet known, very close to the boundary between sub-Jupiter mass and Neptune-mass at 0.1 M. This planet orbits very close to the star.