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.
Island
Greenland is the world's largest island, with an area of over 2.1 million km, while Australia, the world's smallest continent, has an area of 7.6 million km, but there is no standard of size that distinguishes islands from continents, or from islets.