Deleted

Despite being based on Chrome, Steam's Browser is not customizable with extensions like adblocker. As a workaround you could use your hosts file to block those domains. You just have to find updated lists of ad-serving domains.
The rest of your questions cannot be answered by me, I never used GameKit and probably never will.

Why does anyone want to use that anyway and not a normal browser?

I've used gamekit a few times and got a couple games from it, they seem legit, its just weird that they require your address if you wanna purchase something. The site requires you to play free to play games and get to a certain level or something like that to get points, some browser games some downloaded games like warthunder.

As for adblocker, you can use borderless fullscreen and alt tab and use a normal browser like firefox/chrome/opera/etc with an adblocker
Don't think there is a way to install adblockers in steam browser

Yes, GameKit is legit. Be very patient when you order something though because their delivery time is 3-4 weeks but personally it always took longer for me.