Which is better for gaming?

The C3. It has a 120Hz native panel, VRR, G-Sync and FreeSync, a 0.1ms response time and four HDMI 2.1 ports. The A2 is limited to 60Hz and has no HDMI 2.1.

Is the bigger 77-inch A2 worth choosing over the C3?

Only if screen size matters more than everything else. The A2 gives a much larger picture, but the C3 wins on processing, HDR, audio, gaming and ports.

Which has better picture quality?

The C3. Both are OLED with the same contrast, but the C3's α9 Gen6 processor and Brightness Booster produce a brighter, more detailed image, and it supports Dolby Vision.

Does the A2 support Dolby Vision?

No. The A2 handles HDR10 and HLG only. The C3 supports Dolby Vision, HDR10 and HLG.

Which is better for a bright room?

The C3. Its Brightness Booster and newer processor hold up better in daylight. Both OLEDs are weaker than bright LCDs in a sunlit room.

Yes. Both use LG's webOS smart platform. The C3 adds built-in Alexa and Google Assistant plus AirPlay support.

LG A2 77-inch OLED TV vs LG OLED evo C3 65" 4K TV

The LG C3 is the better TV for most people. It has a 120Hz panel, the α9 Gen6 processor, four HDMI 2.1 ports, Dolby Vision and Atmos, and 40W 2.2 audio; the A2 has a 60Hz panel, the older α7 Gen5 chip, three HDMI ports, 20W 2.0 audio and no Dolby Vision. The A2's one advantage is its 77-inch screen, which gives a much larger picture for a big room. Reviewers and owners praise the C3 for gaming and HDR, while the A2 is seen as a step-down OLED with a big panel.

Choose the LG A2 if you sit far from the TV and want the biggest OLED picture for a wide room, and you don't game or need 120Hz.
Choose the LG C3 if you want the better picture processing, 120Hz gaming, Dolby Vision and Atmos, and four HDMI 2.1 ports in a 65-inch size.

Prices

LG OLED evo C3 65" 4K TV: $62.99 at Westside Boot and Shoe (checked today)