It appears to simply be a solubility issue not a precipitate. The powder as I read is slowly soluble in water. So, hot tap water would be the best way to dissolve it. Also whether you use hot or room temp water sift it into the water slowly while mixing. If you simply dumped the powder into water then that's likely the issue.
I always get the tap water as hot as it'll go then fill my glass jar, put on the lid, then swirl to create a vortex. Then I set it down (not to disturb the vortex) then add my EI dosing salts. I re-cap the jar then invert the jar 4-5 times (not shaking it but rather inverting back & forth end over end). Then recreate the vortex and within 30sec my salts are dissolved and that's for my 135gal tank.
Now try the aforementioned dissolution method for your ferrous gluconate but understand that you need to add say a tsp or two at a time. Mix that until it's dissolved then add the next dose into the jar in the vortex. If you do not have a jar then note that a mason jar with a lid or a spaghetti jar with it's lid works great for this.