By a long mile, the most impressive and ambitious game I've seen done with GameGuru both visually and mechanically was "Space Losers" by "Dimoxiland", but I think the developer stopped working on it quite some time ago as their YT channel went silent over 3 year ago.
That is the only GameGuru project I've seen to date that I would rate an 8 or higher by Steam standards on first impressions, and is one of the rare instances where the engine's limitations are holding back the capabilities of the developer. (Dark Skies as Wolf mentioned, would be another case of this).
https://www.youtube.com/user/Dimoxiland/videos
Dark Skies, while impressive, still suffers from GameGuru's horrendous AI and combat limitations, and the graphics do look very outdated (which again, is the engine's fault and not the developers). Space Losers addresses both of these issues by (1) creating enemies that use an entirely different AI and style of combat, and (2) use simple "cartoon"/low poly graphics which I believe the engine is best suited for.