Yes, as a public agency and as per RCW 39.12 a firm must pay at least the prevailing wage if performing a "public work" with Grant County. If your company is planning on performing "public works" for any public agency, please see the "Contractor Training Page" at the Washington State Department of Labor and Industries. https://lni.wa.gov/licensing-permits/contractors/training-workshops
This page and others you will find while visiting their site contains many resources for a contractor to understand the various requirements for performing public works. These include topics such as certified payroll requirements, contractor training requirements, prevailing wage requirements, retainage, and more.