I have samsung gravity smart phone it powers on but the screen won't respond to touch the...

Your touch screen could be bad or bad software. I would try a hard key master reset first. Here are the instuctions to do the master reset for your Gravity Smart.
Back up all necessary data on the internal memory.
With the phone powered off, slide open the phone.
Simultaneously press and hold the Power key and the T key on the slide-out keyboard until the Android system recovery screen displays.
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Press the Volume key down to scroll to wipe data / factory reset.
Press the Power key to select.
Press the Volume key down to select Yes - Wipe user data.
Press the Power key.
Press the Volume key down to scroll to Reboot system now.
Press the Power key to confirm.

I would try a master reset first, its a long shot but you can give it a go. If it still doesnt work then I would look into a warranty exchange.