Cuban/Tropical gar. more likely to be able to hold it's own against an asian aro (unlike the Lepisosteus gars) yet not big enough to pose as a thread (unlike gator gar). are asian aros aggressive?
I'd attempt a Florida gar of approximate size to the arowana... if they start off small and at the same size then it might work. If the arowana is already large then I'm not sure I'd risk either fish by introducing a gar.
my florida did not tolerate the silver I tried on more then one occasion to house with it. They took turns hammering eachother every-time I tried to house them together.
If you try have a hospital tank ready to split them if need be. the combo did not work for me.
But I would try either floridas or maybe a cuban. I would def not try a gator. not unless you have a large pond to house it or similar gallonage. eventually it will outgrow the arrowana. I wouldn't trust any gar with a fish 1/2 it's size or less.. let alone w/ the shark-like mentality of the gator gar.