The operator is not mapped by default. Set it like this:

nmap gs  <plug>(GrepperOperator)
xmap gs  <plug>(GrepperOperator)

This maps the operator for normal and visual mode.

In normal mode, the operator gs takes any motion and uses that selection to populate the search prompt. The query is quoted automatically.

Useful examples are gsW, gsiw, or gsi".

In visual mode, it uses the current selection.

References:

:h grepper-operator
:h operator
:h {motion}
:h text-objects