Isn't this just voicemail? There are even services that let you leave a voicemail directly without actually calling the person. How is this different/better than that?
VoiceGem is for times you have a story to tell, but you don't need the other person on the line right that moment. You can send or reply to one whenever you want. I have received many great jokes or stories from folks in my VoiceGem inbox over the last couple days. That hasn't happened with voicemail before.
That said, isn't there already a very popular app that does this?