I assume this is simple signal.
You have the GPS. It has a data sheet. There is no reason to assume anything.
Based on my research I think I'll be able to pull GPS values for speed
If you don't even know what kind of signal it is, I'm not sure how you reached this conclusion.
On the other hand, most GPSs output serial data, where the current position is a string. Knowing where you are, and where you were, and the difference in time between there and here, you can compute the change in location and the speed.
however how to convert that into signal that I can connect to speedometer is a mystery a bit.
Without knowing anything about your speedometer, yes, it is.
I was thinking of replacing hall sensor
Why? That system will work anywhere, at any speed (though I suspect that your assumption of one pulse per revolution is wrong). A GPS works only outdoors, and only gives reasonable results when you are moving fairly quickly.