You may want to look at the codes in the a programin[/url]g for some help in integrating the gps data and stearing into the arduino. the project is aimed at rc airplanes but it might give you some directions to consider. There are examples of gps parsing there too. I know there is a lot of ground based robots running the code.
I would love to see more details of your system, ie valve control interface etc.. 
Jay