I've used all three - WebKitGTK, Electron and also QWebEngine:
- Electron was better suited for a full screen multimedia experience 'for fun' for someone, to be archived onto an optical disc.
Since it's offline only, it was easily distributable as a binary, cross-platform and shouldn't break in future. - WebKitGTK because ubuntu-mate-welcome used it, and a very old version of my app started off like a web app. A natural fit for a GTK environment. Though, it was painful when users got a 'white screen' due to hardware acceleration bugs.
- QWebEngine because some PyQt applications I've written benefit from using web code. A natural fit for Qt environments. My personal choice.
Even though Chromium isn't an ideal browser for me (mostly due to poor UI), Blink does perform better and supports more APIs/specifications then WebKit. I found browsers like Epiphany to feel jittery (maybe lacking hardware acceleration). Historically, WebKitGTK has had some maintenance problems. Even Qt removed their QWebKit.
Personally I'm all for native app experiences. In your case, if you're building a HTML game, probably:
- Electron, if cross-platform or performance/support is important.
- or WebKitGTK, for being lightweight and aimed for Linux/GNOME users.