The ECU will likely have a speed sensor input. Mount the GPS unit on the dash somewhere so it sees the sky.
Give it some ignition power and the output goes to the speed sensor input.
The ones I bought give the right number of pulses to be accurate for an LS1 ECU.
But the research I did says the ECU doesnt actually care what speed it is doing, it just wants to know if the vehicle is moving.
I have no idea whether or not that is the case with the Haltech though.
If I was in your position though, I would be ringing Haltech to try to get some advice.
If you have the right questions, most places, regardless of industry will give you some good info.
With the oxygen sensor, any data collected should give you good AFR readings.
Again, with the research I have been doing, I have found that the AFR data is the most important data to read.
Getting it wrong does one of 2 major things.
It can increase your fuel consumption way beyond what it should be.
Or, you will be driving around with a potential hand grenade under the bonnet.
Cheers
Rob