We have never tested on the BBB. But in theory, it should work.
The gyro values will always increase or decrease, even when the IMU is not moving. The is the drift as a result of noise all gyros produce. This is why a complementary or Kalman filter is used to fuse both the gyro and accelerometer readings for an accurate reading.
The angles your are interested in above are CFangleX and CFangleY (CF for complementary filtered). Do these angles seem correct when you rotate the IMU?
Mark --OzzMaker.com --