What would I need to run a .gw file?
As far as I understood, you would need a browser running the gw app. Or maybe you meant a black box that is called gw and is actually a browser with a filter to unpack the .gw file and just run it.
If so, your arguments should be compatibility. You could make a testable environment. I recently tried to play a game, which would not work at all. And I tried with two different browsers.
Another argument would be smaller distribution size. From my perspective, a lot of devs pack a 100MB browser for their 5MB game, so that it will run offline. It is less of an overhead if you consider big games, like a 1GB renpy game that comes with 100MB renpy files that are always the same. But even there you need compatibility. And also for that 5MB game. Maybe it runs on a browser made in 2024 by using quirks of that thing, and one cannot guarantee, that the same quirks will work in 2028. So just packing it with a tested environment is just the practical way.
It would make a bit of sense in that light. But not if you consider it an alternative. If the game is under current development, it will need to run on current browsers, if it is a browser game. You might have archived versions, that would run with archived gw versions.
If you want to offer a service, I suggest you do research on how actual html5 games are distributed and how they solve problems. If you can offer anything that would be a benefit. The top-sellers of the web games are a good starting point, since those are the games that have proven to be making money.