That those programs are licensed under one of the BSD licenses doesn't have anything to do with Stallman's use of the term "GNU/Linux" and recommendation that others join him in using the term "GNU/Linux". This is not a "demand" as you claim. Stallman explains his position in brief in https://www.gnu.org/philosophy/words-to-avoid.html#Linux , at length in https://www.gnu.org/gnu/why-gnu-linux.html and there's a FAQ as well.
In fact the specific point you raise has been raised before and is addressed quite well across multiple questions in the FSF's FAQ on the term GNU/Linux:
https://www.gnu.org/gnu/gnu-linux-faq.html#others -- Many other projects contributed to the system as it is today, but they don't insist on calling it XYZ/Linux. Why should we treat GNU specially?
https://www.gnu.org/gnu/gnu-linux-faq.html#distronames0 -- My distro's developers call it “Foobar Linux”, but that doesn't say anything about what the system consists of. Why shouldn't they call it whatever they like?
https://www.gnu.org/gnu/gnu-linux-faq.html#bsd -- Should we say “GNU/BSD” too?
For a while Debian has distributed multiple systems where GNU was the predominant OS atop other kernels--GNU/Linux, GNU/kFreeBSD, and GNU/Hurd. It helps people understand the major components involved by naming things according to what they are. Such naming is also fair to those who ask for a share of the credit for their major contribution to the overall work, as the GNU Project asks people to give them a share of the credit for their major contribution.
>There are distros where the only gnu code is the kernel.
What? GNU never finished Hurd kernel and that is why it is now GNU/Linux because Linux was the missing piece since they could not get Hurd working. In other words, there is no GNU kernel. Please don't just make things up.
Alpine Linux for example replaces libc with musl and gnu binaries with BusyBox.
What, like Hurd with a non-GNU userland? Where? I know there is Debian with Hurd, but that still uses GNU userland.
Gnome is still listed as a GNU package here: https://www.gnu.org/software/software.html
Today, however, Gnome is a rather independent project, with its own GNOME Foundation.
(There was also GNUStep but it was never popular)
Stallman cannot really make any such demands.
Edit: sorry for the confusion, I meant the Linux kernel, which is not gnu.