You know stars have a strong gravitational force, right? And well, turning gasses very heavy just to make them get together with a radial gravity would be simply irrealistic. You should turn gravity OFF, put newtonian gravity (aka: second gravity) ON, and then just make a blackhole, dmnd cover, and put the gasses around it. Not very realistic, but better than gasses which don't act like gasses.
Edit: or above, I forgot how tpt.el 'gravity' command works with the older gravity, so you can just simply use that command and 'done'. However you can use my alternative so you don't have to switch that value to 0 again.
tpt.el.hygn.gravity = 1
tpt.el.hygn.advection = 0
tpt.el.nble.gravity = 1
tpt.el.nble.advection = 0
tpt.el.co2.gravity = 1
tpt.el.co2.advection = 0
tpt.el.oxyg.gravity = 1
tpt.el.oxyg.advection = 0
tpt.el.plsm.gravity = 1
tpt.el.plsm.advection = 0
Sorry bro didnt know i have been experimenting with lua and gasses so im sill working on it...