My favourite C&C game was Tiberian Sun, so I'm really interested in seeing an official mod from OpenRA. However, It's not clear to me if it uses the proprietary Freeware game content (terrain, units, structures) of the original game. Can anyone please explain if that's covered by the GPLv3 as well? Thanks.

No, the original assets are definitely not covered by any open content or free software license. https://github.com/OpenRA/OpenRA/wiki/FAQ#is-that-legal

Probably not, I didn't like it too much. At least compared to Red Alert 1, which is my all time favourite.

And on that topic maybe I should plug redalert1.com, a still active community for the game, and others too.