Tilt sensing is done by the accelerometer, which detects the acceleration due to gravity and the angle that the "down" direction makes with the accelerometer axes.
This is inaccurate if the object to which the accelerometer is attached is accelerating or rotating, so the gyro helps to compensate for the errors.
I agree with Koepel to start with just the accelerometer and see if that works well enough. A good intro to tilt sensing is found on this page, with correct roll/pitch example code. See the "sample sketch". (Note that there are plenty of sites posting incorrect accelerometer tilt code)