Though HTC One from Sprint was advertised as world phone, no working method was available to unlock it from Sprint and use of GSM networks.
Here is guide lifted from Sprint HTC One forums on xdadevelopers.com explaining how to perform SIM unlock on Sprint HTC One and switch to GSM mode. You need have your phone ROOTED to get it SIM unlocked.
1) Root yout HTC One - You can find guides on how to root your phone here at XDA 2) Download Terminal Emulator from Google Play Store here 3) Run Terminal Emulator 4) Switch to super user - type "su" ( excluding quotation marks ) and grant super user permissions when asked 5) Type "am start com.redbend.vdmc/com.htc.omadm.test.TestMainActivity" (excluding quotation marks) which opens a drop down menu on your phone 6) Select API option 7) Scroll Down and Select sim lock on next menu 8) Reboot your phone and now your phone will be SIM unlocked and can accept SIMs from GSM providers.
You need to enter *#*#4636#*#* in your dialer to open hidden menu -> select phone information, scroll down and select GSM sepcific network in “Set preferred network type” drop down list
Here is a video explaining this process.