First of all, congratulations on the program — it’s really useful!
I have a question about maps created with Tiled: which license applies? Because if they were considered “works based on the Program”, they would be covered by the GPL license.
Your maps are your own work and copyright, they are not “works based on the program”. The tileset artwork may have its own licence (again independent from that of Tiled).
To be honest, I’m not sure. As for LaTeX, the GPL license requires that, if the LaTeX package you use is licensed under the GPL, whatever you write must also be licensed under the GPL as an “aggregate.” That’s exactly why I was asking for information from those who know more about the subject.
eishiya’s comment is correct. You got your answer from someone who knows more about the subject 
I didn’t mean to belittle anyone; I was just looking for an opinion from someone who knew the subject well, because after rereading certain comments about the GPL, I had some doubts (I’m by no means an expert on legal matters or licensing issues!).
At this point, I’d say I’ve got my answer!
@eishiya , do you know of any tileset artwork that’s either free or compatible with the Creative Commons licenses?
Tiled’s GPL applies to Tiled and therefore derivates of Tiled. When you make a map with Tiled, no portion of Tiled/Tiled code is in your maps, your maps are not derivatives of Tiled. The same logic applies to GPL art programs like Krita, Krita’s licence page even explicitly spells this out. I don’t think Tiled’s website spells it out like that, but Bjorn, the lead dev, has commented to that effect in the past.
As for tilesets, I can’t personally recommend any as I make my own, but you can find many CC and CC0/PD tilesets on OpenGameArt: https://opengameart.org/