How to set zero using touch probe for Z axis?
If you check the link posted below, you will see that in the image on that website, there is a button called Zero Position. The developer also states in the description of its functions that Zero Position can be achieved using that button.
To find additional information, use the link: http://zapmaker.org/projects/grbl-controller-3-0/grbl-controller-main-screen-options/
Just read the descriptions on the page, scroll to the end of the article to read about the zero position as well.