I don't doubt that it has at least some potential in increasing cannabis yields since it's effects are so drastic, but you would need to use a very very low dose and time it really well, because otherwise you end up with super spindly and lanky growth that is mostly stems and not bud sites.
There is some info to read on forums about combining Ga3 and Jasmonic Acid to increase trich production, but not very much scientific date involving cannabis and hormones at all..
Just know that Ga3 is insanely potent. If you have very pure powder, it takes a ridiculously small amount to make a spray potent enough to cause a plant to look like it was from a different species with crazy growth lol