How linux gain profit if it is open source os as well as a freeware?

Well, in reality, it doesn't by itself. Companies that put out Linux distros (and other open source software) usually charge for service and support. Something to keep in mind is that Linux by itself is pretty useless. Most companies that distribute Linux do so as a "distro" (short for "distribution"). These distros include lots of other software to make it useful. Sometimes, this software is not open source and a company might charge for the distro (which they are allowed to do).