I'll leave it to the chemists to really answer that one. As we've established before, your desire to go down rabbit holes way exceed mine (and I don't mean that in a bad way!
) But from my limited understanding and past reading, it has changed form so that certain tests no longer register it and even more importantly, your plants won't be able to use it -- which is why I, myself, was a little curious of what happens to the buildup of this particular type. I'm remembering that I've read the buildup of the chelates of the other types can theoretically start doing weird things, but I thought this sort of problem was not true for gluconate.
This thread 'may' help or beget even more questions...
Some of the best learning in life happens in the rabbit holes
Great thread and threads linked within it. It's a good primer for understanding the different forms of iron and ways to administer in the tank. It does not directly answer the question of what happens to iron gluconate excess, which I'm going to post directly to Seachem. I think they have a Q&A they respond to. On their bottle they pre-warn about getting a 0PPM test result afterwards and recommend "Due to rapid utilization, test within 30 minutes".
It looks to me like iron has a lot of unknowns in the hobby. I see people post they run 2-3PPM weekly dosing, others much lower, and various guidelines recommending weekly dose of .1 to .6ppm. A "how to use" video I watched for the Hanna HC Iron detector, at ~2:35 the instructor is saying her reading of .6PPM is "deficient" and that she runs at a constant state of 2-3PPM. That would be a very large weekly dose, maybe 10PPM to maintain a constant 2-3ppm. She is in aquaponics, not aquariums so maybe there is a big difference.
For anyone interested in iron tests, the Lamotte strips
appear to work. I've been using them for a couple weeks, comparing to other tests, and doing "before and after" testing. They are very convenient compared to other iron tests which can be so complex, user error will result in them never working (eg. Seachem iron test). Dip for 2 seconds and near instantly you'll see if there is iron, takes 30 seconds to fully develop. It's not super resolved with the 0PPM, .3PPM, .5PPM ... pink gradations but I find it useful enough.