the Gain Reduction Meter tracking calibration is pretty straightforward. The idea is simply to get a given gain reduction in VU (+4) mode and then have the meter track accordingly and show the exact same gain reduction in GR mode.
I've calibrated mine with a -10 dBu signal as advised in http://www.axtsystems.com/index.php?option=com_content&view=article&id=56:1176lncalibration&catid=34:1176ln&Itemid=62
A good idea is to have a precise metering, I've used Spectrafoo, which is absolutely first class in this domain.
So you want to zero your meter with no signal in GR mode.
Then switch to VU mode, 20:1 ratio, fast release, bypass gain reduction, input the test signal, back your output to read O VU.
You should now have signal in = signal out on your metering, so that when gain reduction will be happening you can confirm it is the 10 dB you are expecting.
Switch gain reduction on, you should see a little drop in your output signal. If not switch back to no gain reduction, turn your input up 3 dB, reduce your output to read again 0 VU, so 3 dB down. Switch to gain reduction, observe the drop.
What you want, as the 1176 is a fixed threshold compressor is to have 10 dB of gain reduction happening. To achieve this you need to input enough signal. Repeat these steps a couple of times, increasing your input and reducing your output so that in gain reduction you compress your signal 10 db.
Confirm with your metering that you have signal in = signal in bypass, and then signal in = -10 dB signal out.
Then do not touch your in/out pots as it would alter the state you just created.
Switch to GR mode, you should have a 0 VU reading in bypass and a minus something reading in gain reduction. Adjust VR55 to read 0 VU in bypass, switch to gain reduction, adjust VR54 to read -10 dB in compression. Repeat these steps switching between bypass and gain reduction so that both readings are correct.
Confirm everything on your external metering, soundcard in my case.
Now your compressor should compress 10 db and your meter should show it !!!