The velocity at any point in time is:
v = vo + at
Taking up to be positive a = -g and v = vo + (-gt)
g is just the magnitude of the acceleration due to gravity.
As time goes by vo is constant and -gt grows linearly with time, so the velocity v changes at a constant rate.
Another way to look at this is to use v2 = v1 + Δv.
When the acceleration is constant Δv = a Δt.
So, v2 = v1 + (-g Δt).
By stepping the motion forward in equal time steps you can see that the change in velocity is the same for each step.