How could I get the player movement speed in my mod in km/h. I want to display it in a GUI in the bottom right-hand corner (which I know how to do). Any ideas?

If you still are thinking about this, then I would suggest you store players position in variable

 

then on specifid tick you calculate the distance the player traveled since the last "readout".

 

lets say the tick is every second then this pseudo code should be right.

 

//calculate distance since last reading
diffx = player.x - prev_posx
diffy = player.y - prev_posy
diffz = player.z - prev_posz

//calculate overall distance
distance = sqrt( diffx + diffy + diffz)

//calculate speed
// speed = distance * 60 *60 /1000
speed_in_km_h = distance*3,6

// store player position for next time
prev_posx = player.x
prev_posy = player.y
prev_posz = player.z

Another way to get the distance (in a perhaps more cleaner way) could be like this:

Vec3 lastPosVec = new Vec3(prev_posx, prev_posy, prev_posz);
Vec3 currentPosVec = new Vec3(player.x, player.y, player.z);

// Euclidean distance between your last and current position. 
double distanceTraveled = lastPosVec.distanceTo(currentPosVec);

 

As for handling the gui, I am afraid I cannot be of any help there as I have no experience in that.

If you haven't so far, you should perhaps read this:

http://www.minecraftforge.net/wiki/Gui_Overlay