If I remember correctly, you can't mix straight Ga3 into water.
I believe you need to wet the powder with just the tiniest drop or so of isopropyl alcohol.
This helps break down the powder so it can be mixed as a solution into water.
To get a solution of 250 ppm of Ga3, you need to mix 100ml of the powder measured out
(then add the alcohol to it) and mix that into 400ml of distilled water.
This sould give you a stock solution of 250ppm.
You should also be warned that this solution does not have a good longevity,
and that it goes "flat" very quickly (with under two weeks) even when refrigerated.
Also, I wouldn't soak your seeds for more than twelve hours.