How to reduce the sour taste in gravy?
The primary balancing factor for sourness is sweetness - so gradually adding sugar (plain sugar, rock sugar, honey, palm sugar...) and tasting should yield good results here. "Whereever you add tamarind, you can add jaggery", one well known indian chef tends to say in his videos. 

The combination of strong sourness (vinegar!) and strong sweetness (plenty of sugar!) is not uncommon in chinese (sweet-sour) and italian (agrodolce) cuisines. Also, western tomato sauces almost always have sugar added unless exceptionally good and sweet tomatoes are used.

As strange as it sounds, giving the sourness a bit more depth with vinegar (for anything with indian or thai spices in it, yellow rice wine vinegar is great; avoid distilled or white wine vinegar!) while also sweetening the dish can help here also. You got a sour dish, make it a great sour dish.

Also, make sure your salt, fat (butter, ghee, coconut oil, oil), and bitterness (spices) are balanced.