What is Gearbox Games??

I recently heard a rumor that Gearbox Games will be working on Halo 4. Wether this is true or not is not completely relevant, but i do not understand their role in making a game like this.

When I go to the Gearbox game website, I see HALO: Combat Evolved as well as many of the Half-Life Titles listed in their Credits. We all Know that Bungie and Valve were responsible for Halo & Half-Life, so what does gearbox games really do?

I guess I do not understand the game development piece and was hoping someone could explain it to me...

the connection they have to the Half Life and Halo series is that they did a Expansion for Half Life, and the PC port of Halo...

other than that, they have really only done the brothers in arms series.