Is it possible to swap the stock tires and rims for 15in tires and rims? If so, does it have any other complications like brakes?

15 in tires and rims are stock on the GS. If you get new tires and rims the tire store will have the calculations as to what you can put on, if you're trying for junkyard sets look on some websites to find out the stock size the GS takes then look for that at the wrecking yard. I've personally had 16s on my old one