Hmmm. I wonder why? I can understand there being a size limit to a gas giant, (When it gets big enough, it's not a planet, it's a star!) but I don't see a limit to how many rocks you could pile in one place?
I'm thinking our resident expert might be able to answer you on this. How 'bout it, RA?
Me neither. I can see why it would be unlikely, though, at least with smaller stars like Gliese, because the chances of there being enough rocky material for a Jupiter-sized solid, probably aren't all that high. Jupiter's 318 times the mass of the earth. That's a lot of material.