which window manager am i using?

[msuggs]

Gnome does come with Metacity by default although if your graphics card will support it Mint will turn on Compiz for extra effects instead. You can check this by right clicking on the desktop and choosing 'Change Desktop Background'

Click on the Visual Effects tab.

None = Metacity
Normal, Extra and Custom = Compiz

[msuggs]

But how would i go around changing that?

That would depend on what you want to change it to. Is it just window managers or File managers as well you're after. Which ones?

[msuggs]

You can install many window managers/desktop environments with the package manager. Aside from the default ones in gnome there is xfce, icewm, kde and fluxbox to name a few. Once installed they automatically add themselves to the GDM login manager and you can choose them by clicking on the sessions button before logging in.