Several people have had this one problem with the FY-G3 handheld gimbal of an annoying leaning of the camera to one side after a few minutes of power. This video shows one of the worst cases:
The first step is to ensure that you initialise the gimbal correctly. For the best results when doing this, ensure that the gimbal is as steady as possible. Preferably, you would place it on a table or a solid surface and then turn it on to initialise. When you turn it on, the lights on the motors will flash blue fairly quickly. Once they have flashed purple and the gimbal begins to move to place the camera in the correct place, then the gimbal is initialised. Please see the video below for clarity.
If you have done this and the gimbal is still experiencing problems staying level, then one more thing that you can do is to update the firmware (at the time of writing, the latest version is the 16/10/2014 version which can be found here). If you do this, and then initialise as explained above, then with any luck, your gimbal should stay level.
If you have carried out these processes and the gimbal is STILL leaning to one side, it is most likely that you unfortunately have a faulty one. You should then contact your supplier for an exchange or refund assuming it is still within the warranty period.