I wouldn't be surprised to learn that gluconate has a mild chelating or reducing action that helps preserve the iron in the ferrous state, but in that case I'd expect Seachem to brag about it. Ok, here's my take on this. Btw, I have never used a test kit for iron. If my plants don't look green enough for my taste, I sometimes add a tiny pinch of Fe-DTPA to the tank. This happens once or twice a year, but my tap water has a bit of iron in it.
Chelation is an equilibrium process with a strong tendency to the chelated form.
So a solution of chelated iron will have a very small amount of the iron in the free form, since it is constantly dissociating and recombining. Some people find that when they use iron test kits, they get a low reading right away, but if they leave it a few hours, the solution gives a much higher reading. My guess is that the test reagent is grabbing the iron ions whenever they are released by the chelator, so the immediate reading reflects the level of dissociated iron at equilibrium, but as time passes and the reagents grab more and more of the iron before it can recombine with the chelator, the reading reflects total iron. Presumably the plants take up ferrous ion from the constantly available minute amounts that are not chelated, although I think I've read that land plants, at least, may secrete chelators from their roots and reabsorb the chelated minerals as such. The chelating agent protects the ferrous ion from being oxidized to ferric ion.