Whether you are a theme developer or a vendor who makes money by selling paid themes, you must distribute your themes under GPL only! As WordPress themes are derivatives of WordPress, and as WordPress itself is GPL, the themes cannot be released under a proprietary license.
An exception to this rule is – if you develop a custom theme for a client, it does NOT fall under the category of "distribution", unless your client chooses to distribute it, in which case he would need to distribute it under the GPL license!
One of the misconceptions about GPL is that anything released under GPL must be free! This is simply not true! Just as there are commercial opensource softwares, you can release your WordPress theme under a GPL license and still charge for it if you like; in fact, there are already a number of theme developers releasing premium themes under GPL!
From GNU website:
"When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for them if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs, and that you know you can do these things."