Mercury is believed to possess an iron core, whereas Jupiter is believed to have a core of metallic hydrogen. (Liquid hydrogen compacted by the immense gravity to metallic state)
I would think that if a gas giant’s mantle were stripped away by the solar wind, the metallic hydrogen core would return to a liquid, and then (In close proximity to the Sun) a gaseous state.

From Spaceref.com http://www.spaceref.com/news/viewnews.html?id=151
“Some astronomers speculate that Mercury is the remnant of a larger planet that was inflicted with a large impact early in its history which removed its outer mantle, leaving the metallic core behind.”