Sure.
So both the Game and PC-mode on the LG TVs have the exact same amount of input lag (I assume this is still the case with the C9). The problem with Game mode on C7 and C8 is that the color gamut setting is locked to wide, which produces far too saturated colors. And we don't want that.
This is not an issue any longer on the C9 as LG now allows you to change this setting to Auto which is the correct setting. In fact, on the C9 you can now properly calibrate the game mode with a proper white balance control and what have you. It's really quite nice, compared to the previous models

The reason why you would still want to use PC mode is for RGB444. Basically displaying the entire resolution for all colors. Without PC mode red text for example can look really blurry, especially at lower resolutions. You be the judge if this is something that bothers you.
If you choose to just use the normal game mode, and you feed the TV a HDR signal it will automatically switch to an HDR picture profile. Here you should choose game as well.
The annoyance with using PC mode:
If you use PC mode for SDR (standard dynamic range) and you feed it a HDR signal the following things would happen:
C7: Very undersaturated colors, reds look brown.
C8: Correct colors, but MASSIVE color banding.
Which is why I mention that you manually have to change out of PC mode when you use HDR signals. I'm not sure though if this is still needed on the C9. The sensible thing would be for the TVs to just automatically switch out of PC mode, but they (LG) chose the above solutions instead, which was a bit annoying.
Long story short. If you don't care for RGB444, then just use the normal Game mode and mimmic the settings I use in ISF mode for the most accurate picture without a proper calibration. And setup the HDR game profile separately. This way the TV should automatically switch between the two modes according to the signal you feed it, and you never have to worry about it.
If you want the best picture then use PC mode for SDR content, but you may have to manually switch in and out of PC mode depending on your signal type. Unless LG have implemented some kind of auto switching.