Yes, you can just run two wires (positive and negative) directly to the battery, but:
1. Make sure the positive wire has an in-line fuse in it, or is fused somehow in a different way. If you chafe or or otherwise damage the positive wire, and this touches the frame, then you create a short circuit which is not protected in any way. That could damage the battery but most commonly will melt the positive wire. Hopefully before your bike catches fire. Not good. So anything that is connected to the positive terminal of the battery should be fused with an appropriate fuse. (What is appropriate? Divide the wattage/power by 12 volts, and round up to the nearest available fuse. So a 20W device needs a 20/12 = 1.7A fuse, rounded up to a 2A fuse.)
2. Determine whether you need "continuous" or "switched" power. Continuous means it's wired directly to the battery, and is also "live" when the bike is turned off. So if your bike is parked on the street anybody passing by could use your bike to charge their phone. Probably not what you want. Plus, if you forget to unplug your GPS or phone, then it will continue to drain your battery. Again, maybe not what you want on the handlebars. "Switched" power is connected to a power source which is only live when the bike is running or when the ignition is turned on. This is generally recommended for external sockets, and for places where you may forget to detach devices. The most convenient source of "switched" power is the heated grip connector which lives somewhere behind the radiator, but there are other solutions possible as well.
If you want to solve both problems properly, then you should get a fuse box. This is a device which is connected at one end to the positive and negative terminals, plus some source of switched power to activate the internal relay. At the other end you'll find up to eight output sockets. Each of these outputs is individually fused (taking care of problem #1) and something like five out of the eight output sockets are switched (taking care of problem #2). You can then connect just about anything to this fuse block, without the risk of overloading existing circuits or anything.