Two Black wires and 2 red wires behind outlet
You can map out all the circuits and count up the number of outlets and lights on this one to see how many there actually is, but it might not be a big deal.  Older homes tend to have fewer outlets because they didn't have as complex codes back then.  My parents have a similar house, and I think all the lights and outlets are on only 2 or 3 different fuses (not even a breaker box...)

If the house was built in the 50's and all the wiring is original, I'm surprised you can tell what color the wires are.  I don't know all of the "standards" back then, but it would seem they used red for neutral in at least some places in your house.  You should open up the breaker box to get a better idea of whats going on (with the power off) and also test the wires in that outlet box to make sure that black isn't neutral.  Over the years some DIY'er could have messed things up even more.

GFI can be installed without a ground, but again, make sure you know which is hot and which is neutral.  Also, I wouldn't wire up "the rest of the house" to the load terminals of that GFI outlet, so you may have to wire nut your reds and blacks and pigtail off of that to go to the outlet in the bathroom.  That extra wire and the extra size of the GFI outlet might also mean that you need a deeper outlet box - don't try to cram everything in there, especially with old wires that will have brittle insulation.