Is gravitational energy usable and could it be depleted? Let's take for example a tidal stream generator. We know that tides are a consequence of the Moon gravity, we know that mass is connected with gravity, so is this mean that 
(1) we use Gravitational energy and 
(2) it could be depleted and 
(3) by using gravitational energy we reduce mass? 
(4) Taking into account this information on a large scale, could our Universe lose all its gravitational energy in this way?


*

*by saying "deplete", "lose": I mean converted to other types of energies.

*let's neglect that the system Moon-Earth is isolated (but water is still a liquid and other properties are same :) )
 A: In the case of the tidal stream generator, you are not using gravitational energy as such, you are exploiting the kinetic energy of the Moon moving relative to the Earth; gravity is simply acting as a link in the system to share some of the Moon's KE with the sea water. That source of energy is indeed being depleted. Even when we do not harness the tides for power generation, they are taking KE from the Moon, which is lost as heat through friction. The Moon's rotation relative to the surface of the Earth will eventually slow to a stop, and it will end up in a stationary orbit. 
A: Strictly speaking in classical mechanics gravity is not energy. It is a force. Being a force gravity can transfer energy to things, like giving an object falling from a height $h$ near the surface of the earth kinetic energy of $\frac{mv^2}{2}=mgh$. But some external source (maybe you) had to transfer energy of $mgh$ to the object transferring energy to gravity in the first place. So the energy didn’t originate from gravity.
So as @Marco Ocram already pointed out the energy of the tidal stream generator didn’t originate from gravity. Gravity is the agent transferring energy. So bottom line- in classical mechanics gravity is not a source of energy to be depleted.
The above said, general relativity predicted, and has recently been confirmed, that orbiting bodies generate gravitational waves. So even if there were no dissipative losses such as friction, the loss of energy due to gravitational radiation could, for example, eventually cause the earth to fall into the sun. However, according to a Wikipedia calculation, the total energy (kinetic + gravitational potential energy) of the earth orbiting the Sun is about 1.14 x 10$^{36}$ Joules of which only 200 J/s is lost due to gravitational radiation. That equates to a decay in orbit of about 1 x 10$^{-15}$ meter per day or roughly the diameter of a proton.
Hope this helps.
