With the GPL (if this is a library) you can require anyone using the the code to also GPL their work or put it under a license that is compatible with the GPL (eg. BSD 2-clause).
From what IDW said in his 2nd post this seems to be his intention (free license for open source projects, commercial license for closed source projects), probably he mistakenly used "non-commercial use".
From what you want I think the GPL is not the best choice, probably the AGPL would fit your business model better. Although TBH the distinction between them can be a legal gray area so if you looking to a lawyer for advice maybe mention this.
Good luck with this, it seems like an interesting idea.
Thanks! The AGPL/a second (proprietary license) seems like the perfect licensing model for my business! Haven't heard back from my lawyer yet, but I will let him know about this licensing option!