Hybrid Wars [Official Site, Steam] is a rather good looking futuristic top-down mech shooter that has now officially released, and it has day-1 Linux support.
It seems GOG does not currently have the Linux version.
Visually it looks quite impressive, I like the fact that you're able to choose your vehicle but I can't say much more as I haven't been able to try it yet myself.
I don't have any contact with the developers for a key so it's likely we won't be giving it any in-depth thoughts. I've hunted on their site for contact details, but they don't seem to have a way to get in touch apart from Facebook.
About the game (Official)
Hybrid Wars lets you command and upgrade your own arsenal of futuristic mechs to take on wave after wave of enemy forces. Jump into a classic top-down shooter experience as you obliterate over 50 types of enemy and team up with friends to cause more mayhem. Harness an arsenal of diverse machines—tanks, robots, helicopters, and the mighty titans—to rise to the top of the food chain. With 8 huge locations to explore, over 150 objectives to complete, and multiple game modes—from single player story to cooperative — Hybrid Wars immerses you in a future where those with the quickest wits and biggest guns survive.