EcoComplete is generally designed to be used without another substrate on top but I dare say you can top it with gravel.
I'm not 100% sure on depth for Eco, but with Tetraplant complete substrate (which is designed to be topped by gravel) they say about 2cm of tetra and 3 or 4 cm of gravel.
I would think you need at least 2cm eco though.
Too quote a webpage I found after a quick google search:
Instructions:
Use 1-2 pounds per gallon of aquarium water. Two pounds or more per gallon will deliver best performance. An ideal depth is at least 3".
It is designed to be a complete substate but may also serve as a bottom layer with your choice of non-carbonate substrate on top (do not use marble, dolomite, limestone, crushed coral, shells aragonite etc.).
For new aquarium set-ups simply pour in the entire contents of this bag and fill aquarium with de-chlorintated tap water or, for best results, de-ionized or distilled water. It will automatically segregate into 2 distinct layers with the finer material on the bottom.
Hope that helps