A software platform is a largely deployed collection of software, such as the Linux kernel and the GNU C Library, which can be used to run software application programs.
The most basic GNU/Linux platform can be as simple as the Linux kernel, the GNU C Library and a shell like bash.
The Graphical GNU/Linux Platform
Most GNU/Linux desktop installations come with a desktop environment like LXQt, Xfce, KDE Plasma or GNOME who all run under the Xorg display server (and, in GNOMEs case, Wayland). Linux+Xorg is the most common denominator for a GNU/Linux desktop software platform.
Some GNU/Linux desktop software require graphical toolkit libraries like GTK, Qt or the KDE libraries. Those programs will work under any desktop environment as long as the libraries are present. Interoperability specifications for the graphical GNU/Linux software platform cover many areas of interest.
The GNOME software platform
GNOME begun deviating from the general GNU/Linux graphical software platform around 2018 when immature developers who design "apps", not applications, got the idiotic idea that GNOME is somehow separate from the rest of the Linux ecosystem. This lead to GNOME "apps" being developed in a way which makes them incompatible with other desktop environments. GNOME Music is an example of a typical GNOME "app" which is utterly useless outside of GNOME (to be fair, most GNOME "apps" are useless in GNOME too).
Choosing The Right GNU/Linux Platform
Linux has a <1% overall desktop market share and the desktop segment is shrinking in favor of mobile operating systems. Mobile hardware is typically locked to either the Android or iOS operating systems, you can install a GNU/Linux distribution on any desktop or laptop computer but you can't install it on any random smartphone. Designing GNU/Linux desktop environments to be suited for smartphones / tablets / smart-TVs first and declaring that your toy desktop is a "platform" is foolish. Designing applications to only run on such a platform is moronic.
Developers of software applications are wise to ensure that they can be used on any and all GNU/Linux desktop environments as long as the toolkit libraries are present. That ensures that the whole <1% of desktop users who run some kind of GNU/Linux desktop can use the software.