Put it in a 5g bucket and when its full, regenerate it.
I would think all the salt in the GFO would be bad for the soil. I would check the pH of your soil, if the pH is too high then Fe will be locked out. It's more than likely that GFO can raise the pH of soil, it precipitates CaCO3 which may leech into acidic soil, raise the pH and lock out Fe.
You can regenerate GFO by running sodium hydroxide through it for a day.
Ya, you just have to rinse all the NaOH out. Good luck finding pure NaOH locally, ever since methamphetamine makers figured out that some drain cleaners were pure NaOH you cannot find it anymore. If you try and by a whole ton of lab grade you get asked a million questions lol.
If you can find red devil lye, it's pure NaOH.
Edit: Link
Sodium Hydroxide is caustic stuff. Just google NaOH burns (or don't).