How to do Finger Tracking which is occluded by an object?
Use a hand model (using say the kinematics) in order to hypothesize the pose of the articulated movement. For example you can look into:

http://cvrlcode.ics.forth.gr/handtracking/

http://users.ics.forth.gr/~argyros/mypapers/2012_06_cvpr_twohands.pdf

http://robohow.eu/_media/special/bib/isuvr13.pdf

After robustly tracking in presence of the occlusions, you could use a machine learning method such as HMM to recognize the gesture.