The velocity at any point in time is:

v = v_o + at

Taking up to be positive a = -g and v = v_o + (-gt)

g is just the magnitude of the acceleration due to gravity.

As time goes by v_o 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.