Single-unit licensing

The code is yours and you are free to do what you want. The only question is why bother releasing it under GPL if you also want to sell it? The code is original and as such yours to do with as you please. There isn't any pre-existing GPL code or other reasons why you would have to use GPL so the only reason to release it under GPL would be because you *want* to release it under GPL. If you want to sell it then why bother with GPL, just sell it.

You seem to be making life more complicated for yourself. Is there some reason why you want to do it this way?

Well, I'd prefer to do it as GPL. But if $LARGE_COMPANY came by and said "We'd like to use your code, but we can't abide by the terms of the GPL license, would you like some money?" I think I'd go for it.

Thought experiment, not actually expecting this to ever occur.

OK well just because you release something under license X today that doesn't mean you have to continue releasing it under license X tomorrow. You can change your mind at any time because the code is yours. If a company wants to pay you for something that is available free under GPL that is up to them and you are free to take their money and sell them the code under a different license.

The one proviso is that neither you nor the company in question can "withdraw" versions of the code previously released under GPL. Anyone with the GPL version is free to use and or distribute (if distribution is permitted) the code under the terms of the license. In addition the company is not allowed to distribute their copy of the code (either under GPL or any other license) unless the license you sold their copy under specifically permits this. Just because you have released a version under GPL does not give them the right to release their non-GPL copy under GPL. - Of course if the two code sets are identical this is a pointless legal distinction because they could simply get a GPL copy and distribute that.

Okay, that's pretty much what I guessed. Thanks.