Technical Details from Each iPhone Model’s Perspective
The A1428 model (Canada and US AT&T network) is GSM. The A1428 supports UMTS/HSPA+DC-HSDPA (frequencies 850, 900, 1900, 2100 MHz), GSM/EDGE (frequencies 850, 900, 1800, 1900 MHz), and LTE bands 4 and 17.
The A1429 model comes in two different flavours, one CDMA and one GSM:
The A1429 GSM model supports LTE in Asia (e.g. South Korea, Singapore, Hong Kong, and Japan, too) as well as LTE in Australia, Germany, and the UK on bands 1, 3, and 5.
Bottom line is this: if your iPhone 5 is a GSM model (whether A1428 or A1429), it will work with GSM networks around the world. However, the super-fast LTE connectivity will happen in the region your phone is designed for (whether North America or Europe/Asia).
Note: when you’re buying an unlocked iPhone 5, you should be buying at full price (=$600-$1500 US$ depending what country you’re in). If you’re paying a subsidized price (normal in the USA), then it’s very likely the phone’s not unlocked.