Chelated vs. gluconate form is mainly an issue of how often we are willing to dose and whether we believe (can't be proven) that having constant plant contact of nutrients is significantly beneficial. You can read about the differences between ferric and ferrous forms throughout TPT and, if I recall correctly, on Seachems website. Plants take up the ferrous form (e.g.; gluconate) almost immediately because it's so easy to do so in that form and you should dose a little heavier than you would chelated forms, because of this. A good way to determine the ideal amount of iron gluconate for your setup is to dose so that you can read some iron (I like the Nutrafin/Fluval test kit) for 20-30 minutes after dosing, at which point it should fall to zero.
I dose .15ppm gluconate (Seachem's) daily and have been for years, but this is for two reasons: 1) I have a Level 1 UVS which rips chelated metals apart rapidly (neutralizing the chelation benefit) and 2) I have an autodoser so I don't have to bother dosing every day. I believe (no proof) that a daily dose of all micros, in the right amount, is plenty to avoid plants running out of nutrients and creating mobile/immobile issues or deficiencies. If I did not use a UVS and dosed less frequently, I would use DTPA (EDTA is only good below 6.5 pH), probably combined with gluc.