If you want to keep what you have, you'll really have to take it up and put an aquatic soil underneath it, so it won't really matter what the plant soil looks like -your gravel will cover it. As long as the gravel on top is of larger grain than the soil beneath, it will stay there. I have used Fluorite for many years, however, and it is very similar to what you have pictured.
Alternatively, you could use your current gravel as a planting substrate (assuming it is not TOO big and is of typical aquarium gravel size) and use root tabs that you insert adjacent to rooted plants, so the nutrients are available to them in the substrate. Also, you can stick to plants that are not root feeders, like Anubias sp., java fern/moss and hornwort, for example.
Ideally, though, you'd start with a nice quality plant substrate, but you'll have to start over with your tank, and carefully put your gravel atop it AFTER you plant your plants, so the planting process does not mix everything up again.
