First of all, gallium is a lot more expensive than salt water. It's also solid at room temperature, so you'd need to wait until a hot day and make sure the liquid metal was entirely molten and not lumpy or otherwise semisolid. Then, you'd want to partially submerge your dice in the gallium so that they are floating with the substance both above and below them for best results, which has two huge problems. First, you can't easily adjust the density of Gallium. Second, you can't see a die submerged in gallium because gallium is not transparent. Neither of these problems is easily resolved.
This means we will, of necessity, be forced to use the less accurate version of the salt-water test, with the dice floating at the top of the liquid rather than in full suspension. Since the differences in density between the atmosphere and gallium are even bigger than between air and sea water, and because gallium at near-room-temperatures flows like molasses, this is basically just not going to work. It's theoretically possible to use it, but the process would need to take hours for the die to settle, and you'd need to use the chi-squared test or something to differentiate between sides, since a simple poke is unlikely to result in the die flipping properly even with significant weighting.
It would be practically impossible to use gallium for this test at all, and even though you (with training and equipment and some modifications to experimental methodology) could, it would be patently false to claim that doing so is easier than using salt water.
Basically, your premise is wrong. It is hard to do the saltwater test, but that has nothing to do with water being too not-dense. It has to do with it being difficult to get the density exactly right for your dice. If you have really dense dice, gallium would be a better choice than salt water, but only because the dice would not float in salt water at all. Really, what you'd want to use is another variably dense solution, probably a different kind of salt. Lye could work well and is cheap, but has obvious safety issues, and still won't float steel or tin or copper dice. Looking through some material lists, liquid iodine seems to be the only sorta-kinda-see-through liquid that's commonly available and dense enough to float a heavy die. It still won't float a solid metal die, though, and it it's only liquid around 20 degrees hotter than the boiling point of water, so there's safety risks involved here, too, plus iodine gas is mildly dangerous (i.e. burns mucus membranes on exposure).
Ultimately, if your dice don't float in salt water, just use math. The chi-squared test is easy and reliable and it'll take two hours tops for extremely high reliability for a whole set.