G2 is GRID TWO: It's the second set of electrodes the electron beam goes through as they are accelarated towards the phophor onthe face of the tube.
Setting G2 on some units is simple. Setting it on others is very hard. Sony 12XX for instance have some nice simple test points where you can meausre the voltage and adjust to get them to the right value. Electrohome's like Nick's you do as Roland has suggested. NEC's have a procedure that takes about 3 hrs to read through in the service manual.....and would probably take a good service technician hrs to achieve.....so I do them as Roland has suggested as well.
I do it by putting up a 0 ire window pattern on the screen then turning contrast to zero and brightness to 10-15. Then look in to the tube faces. You adjust G2's so that you can see the raster start to glow. I E you should just start to see the active area of the phosphor start to just light up. You should check this a few minutes after you've done it as green phosphor has a longer decya time and if you over shoot then turn it back down you will not see the result immediately. Check again after a few hrs. If all is OK you might want to use some tippex or nail varnish to then dab the G2 pots to stop them from moving.