GTK already has JS bindings. If you're running Gnome, then substantial parts of what you're seeing and interacting with on your desktop is JS.
The Gnome folks tried anointing JS as the de jure development language across the Gnome project (i.e., for apps, too) a few years back, but there was a minor backlash. Unfortunately, Electron came along within the next year or so has since snagged most of the mindshare that was up for grabs after they backpeddled a bit. You can write JS apps, but it's not a prominent park of the environment outside of Gnome Shell and extensions, AFAIK. The documentation is pretty lacking.
GJS is SpiderMonkey under the hood—the same JS engine used in Firefox. It would be interesting to see if XS were an improvement for Gnome. Web browsers are subject to constraints that aren't necessarily a relevant factor for static packages downloaded from your distro vendor. For example, it looks like XS/Kinoma/Moddable supports AOT compilation, which could be a win.
Metrics here look like they focus on RAM and not on speed of computation.
On the softer side, GTK folks do favor C, and the explicit endorsement of (L)GPLv3 is worth brownie points in that circle. (SpiderMonkey is written in C++, and MPL2 is compatible with (L)GPL, but its copyleft is weaker and allows more permissive use.)
In any case, this project is really neat. I only wish I'd heard about it back in 2015.
IIRC, it didn't really let up. I don't have a reference, but I vaguely recall something that, if not an official recantation, was akin to the general sentiment of backpeddling from the original stance, although I may be misremembering. In any case, the forthcoming concerted effort to go all-in on JS-for-Gnome didn't really happen as that post suggests it would.
A couple months after the fomentatious announcement linked above, GitHub quietly started work on Atom Shell and then a year later released Atom and Electron under MIT. What a massive lost opportunity for the Gnome folks to take advantage of the lead they'd set up for themselves instead of bowing to the vocal pressure! Had that not been the case, GTK+JS might've been the go-to framework for rapid cross-platform desktop app development in the instances where people are reaching for Electron today. (And that recurring plea for more "native" UIs would never be heard—or at the very least there'd be a straightforward path to migrate codebases to native in a piecemeal fashion if there was any motivation to do so.)
Any technical barriers to this ?