Disclaimer -- I am not a sodium gluconate (SG) whiz, so take this advice for what it's worth. I want to switch to using it, but I have a fair bit of EDTA to use up first, so I don't have personal experience with the stuff.
It appears that you can make up to a 60% solution of SG in water -- as in 60 g SG plus 40 g water. Given many of us are familiar with using 50% solutions of NaOH and EDTA and other things, I'd suggest making a 50% solution instead -- 50 g SG plus 50 g water.
Buuut there's a big problem with this plan --
SG is chemically stable in the absence of microorganisms, but it is highly biodegradable -- I'm reading numbers like over 90% degradation in just a couple of days.
Translation -- IF you can keep the solution sterile, it will stay good for a long time, but if this solution gets contaminated with bacteria and other microbes, it's going to go bad really fast.
So if you want to make a SG solution, I'd use boiled distilled water and I would also include a broad spectrum water-soluble preservative to protect the SG from being chomped up by microbes. Liquid Germall Plus would qualify, I'd think.
But that brings me to another big problem --
How can you tell if the SG has biodegraded? There may or may not be any visual clues -- you might see bacterial slime or fungal growth in the solution or you might not. The first clue you might get that the SG solution wasn't good could be a nice crop of DOS. (The main benefit of using a chelator in soap is controlling rancidity/DOS. Reducing soap scum is only a secondary, somewhat limited benefit. )
My take on this?
Use the powder directly as needed. Or make up an SG solution for the day's soap making, and discard any leftovers at the end of the day.