Sign: gravy / grease
The sign for "gravy" can also mean grease / greasy / oily -- if you do it using a negative facial expression and in a sentence in which the term "oily or greasy" would make sense.
The sign is based on the concept of the juice that drips off of a piece of meat.
The sign can be done using the thumb and the middle finger or it can be done using the thumb and the index finger.
For example, see this video and check out how Helen Booker signs "gravy" at the 2:52 mark: https://youtu.be/5IX8jgDiDts?t=171
Note that she is doing GRAVY with the middle and the thumb.
The fact is GRAVY has several versions and is not differentiated by whether you use the index or the middle finger but rather by context and facial expression.
The dominant hand moves downward. The index and thumb come together. Repeat. The base hand stays put.
Note: compare this sign with the sign for skill or talent
Take a look at the sign for talent / skill at the 1:51 mark of this video: