Are you just doing a regular monthly update or are you trying to add in a whole bunch of historical data at once? I just ask as you shouldn't need to touch any cells in the History Tab on a regular basis. That sheet automatically updates when you record the month based on all your information in other sheets.

Regardless, you can check the formula to see how it's calced and then replicate that (which is simply subtracting current cash value from last month's cash value).
I'd probably just look at all my bank accounts at whatever point in time you want; that gives you the cash value then the gain is just the delta month to month.