The code which computes the innovations and Kalman gain should be visible to you, but are not available through a public API right now. You can inspect the source code to see these variables.
Most of the code you are looking for is in the fusion.internal.IMUBasicEKF and fusion.internal.MARGGPSFuserBase class. These are internal classes so they may change in a future release. For R2018b the computations of the Kalman gain and innovations occurs in the correctEqn() method on line 72 of the fusion.internal.IMUBasicEKF class.
Can you give us an idea of what your application is and why you’d like to see this? That might give us an idea of how we can better support this in future releases.