All phones sold on Apple's online store are unlocked, but they come with an unactivated carrier SIM. I purchased an iPhone 7 (t-mobile), then popped out the SIM and put in my old AT&T sim. No problem. I suggest just buying the t-mobile device and doing the same, if you're looking for a GSM device.

They do sell iPhones in the US not tied to a carrier, but usually not until a few weeks/months after the release.