Can this bike take disc brakes?
 

I've searched this forum and done some research online. I'm pretty sure that I need a new fork (which I want anyway) and an adapter in the back, plus new hubs. But there appear to be mounting holes (one in the front and two in the back) for something. The bike is pretty old. Are these just an older style of disc brake mounting holes?

pretty sure those are just fender mounting holes

Those are rack mounts - disk brakes won't work on that bike.

No. Those are rack-mount holes (or rack-mount 'braze-ons' as they used to be called in steel frame days).

Yes, you will need a disc-compatible fork, a rear adapter like one from A2Z Products and wheels with disc hubs. This will not be a small-ticket changeover; might be time to consider a new bike if you want discs.

Awesome guys, thanks.

dminor: I have thought about a new bike instead... I'm still considering it. But I like the idea of being able to upgrade one piece at a time.

That is a fun idea, but you're going to spend all this money upgrading the components, and in the end you'll still have an old GT frame you'll want a new frame to match your higher quality parts, so you buy a new frame, and half of those parts you just spent a bunch of money on wont work with the new frame....