During this proposed period in the asteroid belt, the main element which contributes to the transformation to gas giant is accretion ( see 'The Protected Zone' and 'Ecosphere I and II' in The Super Earths section ).

I should also re-iterate the idea, outlined in my 'Refined Hypothesis' in 'Overview', that the orbits of these celestial bodies are all linked and change togther, periodically moving outward, away from the sun, the change influenced by the impact of the object which starts the planet's tectonic pattern. As part of this sequence, when Mars moves from its present orbit into the orbit that Jupiter lies in now it will lie for a period within the asteroid belt. The absorption of debris from countless successive collisions increases the mass of its gathering gas clouds. This increases the gravity of the object lying within this region, or orbit, which is crucial in retaining the gravitational equilibrium of the solar system.

This is how old dead Earths are disposed of while still serving a practical purpose by keeping the next Earths on their course within the ecosphere ( the band within which life is possible ). They also protect the inner planets by absorbing debris from the outer solar system.

However, the asteroid belt as it stands today no longer has enough material to make another Gas Giant of Jupiters proportions.

I believe the asteroid belt is the remnants of the enormous disk of gas and dust that is believed to have once surrounded the sun ( see: Super-Earths ). It was much more extensive in the past, and shrouded in gas, which provided the raw material for the gas giants clouds to accumulate ( I believe this is what we are seeing when we observe other systems like Beta Pictoris - see: Beta Pictoris in 'The Super Earths' ). But the asteroid belt is now much depleted by the passage of each successive Earth through it.

The present bulk of the asteroid belt's mass is, carbon, iron, nickel, silicon, and many other 'rocky' elements. The bulk of Jupiter's mass is Hydrogen and Helium. These are the elements that each successive passage have taken from the asteroid belt.

The Gas Giant that Mars will become will be much smaller, or perhaps not a gas giant at all, but simply a larger 'rocky' world ( see: 'Intermediate World' in The Super Earths ) and perhaps the gravitational stability of the system will then no longer be sustainable.