Penumbra Overture to be open source

[Conan Kudo]

Will other companies be able to use the engine to make commercial games of their own?

Since it's under GPL, I would guess no.

Wrong. The GPL allows for commercial games. However, most commercial game companies would not use GPL engines because it means they have to release any and all code relating to the game engine. The GPL also doesn't apply to trademarks and game data such as art and music.

Due to the inertia of the game industry, it is effectively no, but legally it is possible.