Question about License and Examples

[AParticularMaker]

Hello, long time lurker first time poster.

I have a question about the code license of the example games on the download page.
I'm currently developing my first game in Solarus and I have a dash function.
Since examples are extremely limited, I copied the dash from YarnTown and modified it heavily.

I would like to develop this game into something commercial to sell on Steam.
From what I've read on this site I think I can do that, but with the GPLv3 license, but I'm 100% sure.
Note if I ever do end publishing this game, I will be more than happy to publish a free demo that anyone can learn from and use in their games.

Thanks