I will try my best to explain the method I use (and most people probably use) to identify G perms
First know that for almost all PLL's (excluding J perms, I think) you have to match up the 1x2 block with the first 2 layers, the I move onto the color of the headlights
If the headlights and the edge piece between the headlights are opposite colors (ex: headlights are green, and edge between is blue) then it's either a G(b) or a G(d) permutation.
If the color of the edge between the headlights is not opposite (ex:Green headlights, and orange edge in between) then it's either a G(a) or a G(c) permutation.
Now, to make sure you are solving from the correct side, always remember that you match the 1x2 block, then you do a cube rotation(ew) so that the headlights are on the left. I hope this helps
The rest you figure out on your own > : D