FOG Yaw Gyro, transformation to remove earths rotation based on attitude (roll,pitch,yaw,latitude)
Your second "drift" term is not an error term at all, it's the FOG measuring the actual sidereal rotation of the Earth. Some systems use this to get an absolute orientation reference; this is known as "gyrocompassing".

When you're working with systems that have this level of accuracy, it would be better to treat this term for what it is, and do an explicit transformation between the Earth-centered nonrotating coordinate frame (which is what the FOG is measuring) and the Earth-centered, Earth-fixed (rotating) coordinate frame you want for your answers.