Hi, Jenny,
I started with a sprinkle (probably a teaspoon) of chick grit in each 6 inch round shallow bowl right from the first. I had crumbles, and they pretty much dissolve in water, but a little grit to start worked out fine. That was per Storey's Guide to Raising Ducks. If you can get the book, or borrow it, I recommend it.
I think the gro-gel is for their first week. I also have vitamin electrolytes plus mix, which I give them in a ricotta cheese tub with a round hole cut out of the lid, free choice, about once a week. I started when one of mine was looking puny, then decided a boost once a week might be a good idea, at least for their first month.
Oyster shells are not needed yet. Too much calcium may be more of a problem than a help. Your ration should have all the calcium they need at this stage. The grit I use, by the way, is granite. Oyster shell is not a substitute. (I say that because every now and then some get confused about that.)
At the beginning of their third week I began to mix grower pellets (which are fairly small) in with the crumbles. I like to change diet gradually, and I wanted to be sure everyone could manage the pellets. I confess, the first few bowls I ground the pellets down a little smaller, then ground half of them down, then just added straight pellets to a decreasing amount of crumbles.
I also have been adding one part rolled oats (the flattened kind - not instant) to five parts starter because the protein in the latter is 26%, a bit higher than the 20% recommended in Storey's Guide. As with the pellets, at first I ground the oatmeal a bit. Another option would have been to moisten the oatmeal before adding it to the crumbles, but then I would have needed to remove the food more frequently (wet food leads to mold which can be deadly to ducklings).
The grower pellets I most recently bought have the proper protein percentage, so as I change over (adding more of the newer food and less of the other until it's all new food) I can stop adding oatmeal.
For treats, I waited until the middle of their second week, and offer them very very finely chopped greens - (and here, I confess, I splurge for a number of reasons) spring mix, dandelions, chard, watercress. This week we started giving them (eleven of them) about a third of a cup of petit peas, fresh frozen which I thaw before giving to the ducklings.
I give them probably about two cups of chopped greens a day. It seems to be their favorite thing! I give it to them in a two foot long trough, in which I also put about a half inch of water.
Hope this helps.