The Sprint LG G2 has a sin card slot and the ability to be used on GSM network when overseas, if Sprint gives you the code to unlock said device. As I understand it, in order for Sprint yo cough up the code, you have to be a long time customer in good standing and have plans to travel out of the country soon.
The other option is to root your G2 and then use software on a computer tethered to the phone and detailed (technical) directions to unlock the device yourself. I have found a couple posts with instructions for how to achieve an unlimited state of unlock on the Sprint G2 on XDA Developers Forum but I have yet to attempt anything on my Lg G2.