Is there any tour guide at Mutianyu gate?

Yes, you can easily find tour guides near the scenic area. The cost is CNY 200-300 per night.
If you wanna go there from downtown area, why not just join in a tour group with professional tour guide and reliable driver? As I know, TravelChinaGuide provides such private tour. And the prices are reasonable. If interested, you can contact traveltravelchinaguide.com. May help!