While I can not confirm with absolute certainty these facts, from what I've seen is that it isn't sim unlocked, but it does support the following GSM frequencies:
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Current Devices: Nexus 5, HP 14 Chromebook (Data Enabled), Google Glass, (2013) LTE Nexus 7, Logitech Revue, Chromecast
Retired: Samsung Fascinate, Droid Charge, Galaxy Nexus, (2012) WiFi Nexus 7, Asus TF700, Nexus 4