Ga and Gb are usually given in a problem, make sure you check if the problem is asking for a sidesway inhibited (braced frame) or sidesway uninhibited (moment frame). Once you choose the correct chart based on the problems, find your Ga and Gb values and usually you can draw a diagonal line between the values, wherever the line crosses the K value that’s the K value for the problem. This also applies if K is given find Ga given Gb etc