carnage would kick his ass, gr would reform carnage would kick his ass, gr would reform, carnage would rip him apart, gr would reform and so on and so on until carnage got tired and passed out from exhaustion.
GR can't use the penance stare cause it don't work on symbiotes and his fire doesn't burn symbiotes either. it just hurts em. so I'm thinking he'd just have to wait carnage out.
it's never explained as to WHY this is so, but GR tried to use the stare on venom, the reaction that resulted was an immense explosion that knocked GR out.
I'm assuming that symbiotes don't have souls and therefore have no moral ethics to live by, when they kill it is as a means for survival or something like it. But if the symbiotes don't have souls (and i doubt they do) the penance stare shouldn't really do much regardless)
The point is, is that not only will the stare not work, it will backfire on GR and knock him out completely.