Following the rules of English, in the construction “GNU Linux” the word “GNU” modifies “Linux”. This can mean either “GNU's version of Linux” or “Linux, which is a GNU package.” Neither of those meanings fits the situation at hand.

Linux is not a GNU package; that is, it wasn't developed under the GNU Project's aegis or CONTRIBUTED specifically to the GNU Project. Linus Torvalds wrote Linux independently, as his own project. So the “Linux, which is a GNU package” meaning is not right.

We're not TALKING about a DISTINCT GNU version of Linux, the kernel. The free GNU/Linux distros do have a separate version of Linux, SINCE the “standard” version contains non-free firmware “blobs”. If this were part of the GNU Project, it could be considered “GNU Linux”; but we would not want to call it that, because it would be too confusing.

We're talking about a version of GNU, the operating system, distinguished by having Linux as the kernel. A slash fits the situation because it means “combination.” (Think of “Input/output”.) This system is the combination of GNU and Linux; hence, “GNU/Linux”.

There are other ways to express “combination”. If you think that a plus-sign is clearer, PLEASE use that. In French, a hyphen is clear: “GNU-Linux”. In Spanish, we sometimes say “GNU con Linux”.

Following the rules of English, in the construction “GNU Linux” the word “GNU” modifies “Linux”. This can mean either “GNU's version of Linux” or “Linux, which is a GNU package.” Neither of those meanings fits the situation at hand.

Linux is not a GNU package; that is, it wasn't developed under the GNU Project's aegis or contributed specifically to the GNU Project. Linus Torvalds wrote Linux independently, as his own project. So the “Linux, which is a GNU package” meaning is not right.

We're not talking about a distinct GNU version of Linux, the kernel. The free GNU/Linux distros do have a separate version of Linux, since the “standard” version contains non-free firmware “blobs”. If this were part of the GNU Project, it could be considered “GNU Linux”; but we would not want to call it that, because it would be too confusing.

We're talking about a version of GNU, the operating system, distinguished by having Linux as the kernel. A slash fits the situation because it means “combination.” (Think of “Input/output”.) This system is the combination of GNU and Linux; hence, “GNU/Linux”.

There are other ways to express “combination”. If you think that a plus-sign is clearer, please use that. In French, a hyphen is clear: “GNU-Linux”. In Spanish, we sometimes say “GNU con Linux”.