Which is better for a bright living room?

The G5. Its glare-free coating and brighter evo panel keep the picture readable with daylight or lamps nearby. The 65-inch has no anti-glare treatment, so reflections are more visible.

Which is better for gaming?

The G5 for raw speed at 165Hz. The 65-inch tops out at 120Hz but adds Nvidia G-Sync certification, which helps if you game on a PC with an Nvidia card.

Is the bigger screen worth stepping down in specs?

If you sit more than about 8 feet away, the extra 10 inches on the 65-inch matters more than the refresh rate. Up close, the G5's sharper motion and anti-glare win.

Which handles HDR better?

The 65-inch lists Dolby Vision IQ, HDR10 and HLG, so it covers more HDR formats. The G5's listing confirms HDR but not Dolby Vision.

Do both support voice control and streaming apps?

Yes. Both are smart TVs with voice control, HDMI CEC and the major streaming apps. The G5 lists Wi-Fi 6E, which is faster on modern routers.

LG G5 Series OLED evo 4K TV (55") vs LG C2 65-Inch OLED TV

Buy the G5 if you sit close, watch in a bright room, or game at high frame rates: its 55-inch evo panel has a glare-free coating, 165Hz refresh and Wi-Fi 6E. Buy the 65-inch OLED if screen size matters most and you watch in a dim room: it gives you 10 more inches, a 120Hz panel with G-Sync certification, and Dolby Vision IQ. The G5 is the newer, brighter, more gaming-focused set; the 65-inch is the older webOS 22 model, so expect a step down in brightness and anti-glare.

Choose the LG G5 if you want the brightest, most glare-resistant picture and the smoothest gaming from a 55-inch set.
Choose the LG 65-inch OLED if you want the biggest screen for the room and mostly watch movies in low light.

Prices

LG C2 65-Inch OLED TV: $50.63 at Canastim (checked today)