Here's a list of VOIP software programs which all support Gizmo5. Typically it's easy to configure with just 3 bits of information:
Username: [This is your 11 digit Gizmo5 number available on my.gizmo5.com beginning with 1747. Enter it with NO spaces or dashes.]
Password: [This is the password you used when you signed up with Gizmo5.com]
Server/Proxy: proxy01.gizmo5.com

Make a test call to the echo line 1-747-474-3246 to insure your microphone and speaker are working correctly and then you're ready to make or receive calls.