What percentage of gas giant exoplanets are in the habitable zone? Approximately what percent of exoplanet gas giants discovered so far are in the habitable zones of their stars? I hear that most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon.
Of the gas giants, the number of "hot Jupiters" is about the same as the number of "hot Neptunes." A little under 9% of known gas giants are in the habitable zones.

An article by John Wenz has the discoveries thus far collated into a chart:



The claim, "most of them are "Hot Jupiters", orbiting so close to their stars that the heat causes them to swell up like a hot air balloon," is at best a bit of a stretch - the only reference that I found for this was a Gizmodo article about Kelt-9b. The Nature paper, A giant planet undergoing extreme ultraviolet irradiation by its hot massive-star host, that discusses Kelt-9b says, 


  The planet is also extremely inflated relative to theoretical models, with a radius of $\sim 1.9\ R_J$. Poor redistribution of
  heat and radius inflation (both of which are also observed in WASP-33b), have been linked to high stellar insolation, although the exact physical mechanisms remain uncertain.


So they did observe that Kelt-9b had a larger radius than is normally expected from a planet of that mass, and it receives massive amounts of radiation from its star, but it cannot be concluded from that that it is the same as putting a balloon filled with air near a heat source.