Unless you are trying to be disrespectful, you shouldn't call it anything but Linux. The complete operating system which is commonly used on servers and PCs is basically the GNU system, with Linux added as the kernel. The principal API of this system does not come from Linux, since user programs do not communicate directly with the kernel. It comes from the GNU C library, which is a part of the GNU system. It's hard for users to learn the difference between the kernel and the whole system if people call them both by the same name.
Many users are confused.
You can also give the system's principal developer, the GNU Project, a share of the credit for its work. ZD Inc. All rights reserved. ZD Inc. All other trademarks are the property of their respective owners.