How to thicken watery gravy?
Your options are really limited to:


Reducing, i.e. simmering until you have less of a thicker gravy, but then (i) you might not have enough and (ii) you risk overcooking the chicken (and some other ingredients) unless you remove them.
Thickening. 


Corn starch is a common way to thicken but many other starches also work. You could even make a roux of flour and butter, then mix plenty of the gravy into that, and add back in. Depending on your dish, you may be able to use a compatible thickener with some flavour (coconut flour works in some curries, for example) but in general you're best using a neutral starch. I suggest being sparing with it - in my opinion a slightly thin sauce is preferable to an over-thickened cloying one.