This is currently the main game of GTC. It is a free-to-play multiplayer co-op game capable of having up to 4 players in one mission. The game is focused on PvE (Player vs. Environment or a.k.a. Co-op) but has some PvP features like Dueling Arenas in the Dojo, Conclave Battles and more indirectly, Event Leaderboards.
Players take control of a 'space ninja' known as the Tenno in a semi first + third person view. Tenno in the game equip special suits known as Warframes which have 4 unique powers each. More information about warframes can be found at the bottom of the page.
The Tenno were recently awakened by the Lotus who then directs them to battle the less benevolent forces in the futuristic Solar System. The Grineer, Corpus and Infested all want the Tenno dead but they are not completely powerless. In fact, the Tenno mastered the art of the gun and blade which makes them extremely formidable opponents.
Your character will join missions with up to four players in a cell. Game modes so far includes Exterminate (kill everything), Sabotage (destroy the ship's core or mining equipment), Rescue (escort a prisoner to safety), Defense (protect an artifact against waves of enemies), Survival (collect oxygen to prevent the life support bar from reaching 0), Interception (block signals in control-point style), Assassination (kill the boss to get loot) and 2 more game modes that will be changed in the coming updates (Deception and Spy).
There are lots of weapons to choose from- shotguns, rifles, snipers, bows, and almost anything in between for primary weapons. Fancy a shotgun that shoots tentacles? Try the Phage. Or maybe you prefer an old school flamethrower? You can buy blueprints from the market and craft them or directly purchase the weapon itself for real life cash. Some weapons and warframes however have components that drop from missions (namely the Primes from the Orokin Void/Derelicts) and some that drop from bosses (e.g. Oberon from Councillor Vay Hek). Others require clan research to get blueprints, requiring a degree of community effort.
The clan system lacks functionality so far except to provide research weapons and Trading Posts which allows trading between members and non-members but occurring in the clan's dojo. New features are expected in the coming updates inclusive of the Badlands (higher level areas with better loot but requiring clans to build Solar Rail towers and fight for territory) and Proxy Wars (players get to choose a faction and total member points will affect clan advancement in a faction).
Free-to-Play features of this game is not Pay-to-Win. The in-game currency is known as Platinum or plat for short. Most things sold on plat are just aesthetic like skins for warframes, weapons and sentinels. There are cosmetic items for sentinels like wings and tails and syandanas (scarf/cape hybrid) for warframes. Weapons and warframes sold in plat are also obtainable in-game without spending plat but buying it with real life money is faster and more convenient. Plat can also speed up crafting time. Trading enables exchange of platinum for in game items like rare mods which makes plat easier for free players to obtain without the use of real money. Other forms of real life purchases include the former Founder's package (Lato Prime, Skana Prime and Excalibur Prime) and current Prime Access (Sicarus Prime, Ember Prime and Misa Syandana Prime). Some of these are exclusive to the package like the first 3 primes and the syandana but the rest can be found in game. Access to the design council is also included in some of the packages.
Warframe receives content updates quite regularly (about 1 minor update a week and a major update every month or so) and the developers have livestreams once every two weeks to answer player questions and give the latest information on upcoming updates.