I think all the GPS units made for MC use come with, or have available, direct-wiring harnesses. And the Street-Pilot and both Zumos I've had all pulled at 14 volts and did voltage conversion internally.
When it comes to GPSs designed just for auto use, or using a smartphone to deliver your GPS, you're likely to have a USB-based power delivery. If you don't use a USB adapter supplied with the device, listen up: Not all USB power outputs are created equal. In particular you need to pay attention to the available output amps. Many USB adapters are limited to 1/2 amp at 5 volts. And it's not uncommon to have a device that wants a full 1.0 amp. And if the adapter has multiple output ports, the amperage rating typically applies to the sum of the current of all ports.
I have what one might call a "Swiss Army Knife" power adapter I use when I travel. It takes input voltage from 110-240 V AC or 10-30 V DC and provides a mini-USB cable for my Samsung phone and a general purpose USB Type A socket for whatever. However, its total output is a max 1 amp. My iPad Mini eats up that amp all by itself. The unit has some sort of internal protection against overcurrent and would not charge either device if both are connected - getting really, really warm in the process. It will charge both fine if done serially.
I have another adapter I use in my plane that takes 10-30 VDC and provides two USB ports at a total 1.5 amp. This would work fine for my iPad used for ADS-B and Chart presentation. But I also have a combined WAAS GPS/ ADS-B receiver that, were I desire to power it during flight would require 2.1 amp a 5V. This would require a special adapter. The manufacturer provides these as an accessory and I will probably need to get theirs.