It is free to create an account on GFN and test any game you have, for 1 hour per session.
To add to this: you don't even need to install anything either, since they recently made it available through browsers on PC and Linux. You have to use a user-agent switcher though, because it's meant to only work like this on chromebooks.
And that's a smart move on the dev's side. However, it's highlighting my point. Sign up for a time. One hour limit. I wonder what would happen if they said, "Open beta test! Come one, come all!"
I strongly believe this is one of those times in life when people have a wonderful idea, and all of the tools they need to start building it...but they've got pipe dreams about how long it's going to take to actually implement it and are not accurately taking into account the sheer amount of infrastructure it's going to take to support it.
This is what I would expect from GFN when it goes live:
1.) You'll need to "schedule" times you want to play games. Those time slots will be very, very limited. (I'd expect "packages". Morning package, evening package, all-day package, etc. And I expect people will be put in qeues and never manage to actually get into the game.)
2.) Without a fibre-optics connection, you're going to get abysmal performance, response times, and connectivity.
3.) With fibre-optics, you'll probably still suffer from regular issues.
4.) It's going to wind up with an extremely limited library of games that actually work...and many of those are going to be older games or simpler, single-player games. So, something like Halo 1 remastered with scaled down graphics...not Doom Eternal.
5.) Actual in-game bugs and glitches are going to be the icing on the cake, rendering some games virtually unplayable.
The "testing" thing going on now should be viewed as a control group. It's the people building GFN using programs they know will work best, with numbers they know they can manage, specifically testing for issues and what not -- like any "beta test". Once those numbers increase by orders of magnitude, we can compare notes to my points above and see what happens.
And I'd be happy to be wrong! If I'm wrong, that means we have a whole library of games that people can play on virtually any device streamed seamlessly over the internet, whether they own a gaming system or not. That would be a w00t!