Hi @asaatifshahab, and thanks for your query. Could you kindly clarify exactly which SDK, API and Tobii Eye Tracker you are using as the co-ordinate system may vary depending on the use case. That being said, the gaze point variables reflect the location on tracked screen ( normalised between (0,0) and (1,1) ) whilst the eye position is the (x,y,z) location of the each eye relative to the eye tracker.
If by vergence, you mean the average eye position between both in space then it would be sufficient to take the average value not the difference.
If you could provide further details of your setup, we will be happy to clarify. Best Wishes.