For most things in the gravel beds, I just sprinkle seeds.
In the gravel I find direct seeding best for large seeds that can push up through gravel and also for tiny seeds with really high germination rates (like turnips) that you can just sprinkle and they will fall between the rock to the moisture and come up. For slower/high value seeds/plants, I've been known to start them in peat pellets or cotton balls or soil blocks and just transplant into the gravel when they are big enough not to get damaged in the process.
Here is a blog post about what I've done for "net pots"
I have solved my quandary I have made a cloner to clone some of my plant starts , such as basil . The cloner works great and it gives me a head start on the plant before I put it in the gravel bed or net pots . I am going to set up a pay pal account and sell my cloner to those interested. using a cloning system will save $ , no seed to buy , no media or potting mix to buy plant in gravel or net pots without the cost of rockwool and the plant is already hydro or aquaponic ready.