Zig Zag up to the radio tower from Franklin's house, very steep at the top.
Here you'll find the current tracked list of routes included in the GTBike V course repository. Be sure to check the repository out if you have interest in building your own routes, or to submit issues with routes. For general discussion about the mod or routes, come visit the community Facebook group.
For issues with this site, development details, of just to take a look at how it all comes together, this project is maintained on Github. When you stop by, be sure to take a look at the readme for the latest list of attributions and thanks. And finally, these courses all utilize the brilliant GT Bike V mod for GTA V.