So essentially you want the calendar “look and feel” of Gameplan but in the context of your existing theme?
It’s definitely possible to make it appear however you would like, through a combination of template overrides and custom CSS and our Themer’s Guide outlines the basics of this.
Theoretically you could copy across the custom tribe-events templates from one theme to another (and the CSS) but I can’t guarantee it would “just work”: their templates may use custom functions that don’t exist in the target theme and the CSS may use selectors that are invalid if the templates are copied across verbatim.
There’s certainly no reason though you couldn’t study Gameplan’s templates and CSS and try porting it across: I’d anticipate you having to make some changes here and there but that’s what theming is all about 🙂
Good luck!