For the B set I have my cables set for both right and left positive to go to positive in on Gallo, same for left.The Gallo subs, or at least those that are presently listed on their website, have separate high level inputs for the two channels. So I think you are NOT saying that you would connect left and right positive to a single positive input of the sub, but rather to separate left and right positive inputs, respectively. And likewise for negative.
If so, and given that the Gallo subs are self-powered, there is no downside to what you are proposing. Also, I'll mention that you don't have to use heavy gauge speaker wire to connect to the sub, since you would be driving the high input impedance of the sub's amplifier and not the low impedance of its driver. Pretty much any gauge that is heavy enough to not be mechanically fragile should do.
The one thing you absolutely do NOT want to do is to connect left and right positive of the B output to a single input terminal on the Gallo, which could damage your amplifier. Although as I indicated I suspect that is not what you meant.