Thank you very much! Coding a new option seems like a lot of work for me because I have no knowledge of coding 
the sources for a brush:
if you click the 'mix" in the color box, you can mix plain color (foreground/background) or gradient.
here is a use case of my grass brush.
with a gradient option, you can choose specific colors to make the blending more “beautiful” but the foreground/background option can work too.
I think it will take longer for me to change foreground/background color or choose a new gradient color everytime I want to use my grass brush because i use other brushes with “mix” option too. but please take a rest and consider this on another day 
.