When gravity pulls on light it blueshifts or redshifts it, which way around does it go? when light is propagating away from a mass does it get blue shifted or red shifted? And if its going towards a mass whats the effect?
 A: In GR you have to be careful to define exactly what you're asking, so let's define our experiment carefully.
Take two observers stationary at a fixed distance from a spherically symmetric mass. We'll call the more distant observer Alfred and the nearer one Bert. Alfred shines a light of frequency $\nu_A$ towards Bert i.e. towards the spherical mass. When Bert measures the frequency he finds it has increased, i.e. $\nu_B > \nu_A$, so in its travel towards the mass the light has been blue shifted.
If Bert now shines light of frequency $\nu_B$ towards Alfred, i.e. away from the spherical mass. When Alfred receives the light he finds the frequency has been decreased, i.e. $\nu_B < \nu_A$, so in its travel away from the mass the light has been red shifted.
The blue and red shifts in the iwards and outwards journeys are equal and opposite. It should be obvious this is the case, because if Alfred shines light down and Bert reflects it back up with a mirror then when Alfred receives the reflected light the frequency must not have changed. If the frequency had changed during the round trip that would mean energy wasn't conserved.
This effect has been measured experimentally by Pound and Rebka in 1959.
There are lots of ways of calculating the frequency shift, but my favourite is to use the equation for gravitational time dilation:
$$ \Delta t' = \Delta t \sqrt{1 - \frac{r_s}{r}} $$
This tells us that time moves more slowly as you get nearer the spherical mass. If Alfred transmits light of frequency $\nu_A$ this means in one second there are $\nu_A$ cycles. But because Bert's time is running slower he receives those $\nu_A$ cycles in a time of less than one second, and that means the number of cycles per second, i.e. the frequency, has increased. The opposite argument applies when shining the light outwards.
A: Light moving away from a mass undergoes a gravitational redshift.  Light moving toward a mass undergoes a gravitational blueshift.
