To quickly pick it up again, I came across this awesome article http://weblogs.asp.net/wim/archive/2004/04/07/109095.aspx

1. You need to find out the GPS boundaries for your city, left upper bound and bottom right bound.
2. Use the c# function getTileXY(double lat, double lon, int zoom) provided below to calculate the X,Y tile coordinates to help you “retrieve” the maps from google map. Eg. left upper bound tile is x=10, y=10 and bottom right bound tile is x=100, y=100, then you use a for loop to download the tiles in between those 2 bounds, x=11, y=10……x=99, y=98….
3. Now, assume you have “retrieved” all the map tiles for your city, use the same c# function getTileXY(double lat, double lon, int zoom) which work out the x,y tile to find out which tile you are currently at.
4. After finding the tile you are currently at, ie. x=55, y=67 tile, you need to work out the pixel coordinate inside a tile to pin point where you are inside a tile, use C# function getXYPixelFromTile(double lat, double lon, int zoom)

A Map Tile looks like this:

OK, thats the theory, you should be able to build up an application hooks up to a GPS device:

C# Functions:

public static Point getTileXY(double lat, double lon, int zoom)
{
/* Per se earth coordinates are -90?<= lat <= +90?and -180?<= lon <= 180?*/
/* Mercator is NOT applicable for abs(lat) > 85.0511287798066 degrees */
if (System.Math.Abs(lat) > 85.0511287798066)
return new Point(0, 0);
/* latitude (deg) -> latitude (rad) */
double sin_phi = System.Math.Sin(lat * System.Math.PI / 180);
/*
* Use Mercator to calculate x and normalize this to -1 <= x <= +1 by division by 180.
* The center of the map is lambda_0 = 0, hence x = lat.
*/
double norm_x = lon / 180;
/*
* Use Mercator to calculate y and normalize this to -1 <= y <= +1 by division by PI.
* According to Mercator y is defined in the range of -PI <= y <= PI only
*/
double norm_y = (0.5 * System.Math.Log((1 + sin_phi) / (1 - sin_phi))) / System.Math.PI;
/*
* Apply the normalized point coordinations to any application, here the coordinates of
* a 256*256 image holding that point in a thought grid with origin 0,0 top/left
*/
double tileRow = System.Math.Pow(2, 17 - zoom) * ((1 - norm_y) / 2);
double tileColumn = System.Math.Pow(2, 17 - zoom) * ((norm_x + 1) / 2);
return new Point((int)tileColumn, (int)tileRow);
}

public static Point getXYPixelFromTile(double lat, double lon, int zoom)
{
/* Per se earth coordinates are -90?<= lat <= +90?and -180?<= lon <= 180?*/
/* Mercator is NOT applicable for abs(lat) > 85.0511287798066 degrees */
if (System.Math.Abs(lat) > 85.0511287798066)
return new Point(0, 0);
/* latitude (deg) -> latitude (rad) */
double sin_phi = System.Math.Sin(lat * System.Math.PI / 180);
/*
* Use Mercator to calculate x and normalize this to -1 <= x <= +1 by division by 180.
* The center of the map is lambda_0 = 0, hence x = lat.
*/
double norm_x = lon / 180;
/*
* Use Mercator to calculate y and normalize this to -1 <= y <= +1 by division by PI.
* According to Mercator y is defined in the range of -PI <= y <= PI only
*/
double norm_y = (0.5 * System.Math.Log((1 + sin_phi) / (1 - sin_phi))) / System.Math.PI;
/*
* Apply the normalized point coordinations to any application, here the coordinates of
* a 256*256 image holding that point in a thought grid with origin 0,0 top/left
*/
double tileRow = System.Math.Pow(2, 17 - zoom) * ((1 - norm_y) / 2);
double tileColumn = System.Math.Pow(2, 17 - zoom) * ((norm_x + 1) / 2);
int y = (int)((tileColumn - (int)tileColumn) * 256);
int x = (int)((tileRow - (int)tileRow) * 256);
Console.Out.WriteLine(x +" " + y);
return new Point(y,x);
}