GPL libraries for an app [closed]

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This has probably been asked before but I couldn't find an answer on search so here I go. I've tried reading wikipedia but only got confused.

Can I create an app (for free or for money) that uses GPL licensed libraries on the front or back end?

(I would assume that most people are already doing this, just want to check)

Answer 1

Yes, you are allowed to sell products that include GPL code. There is an important caveat however.

If you create an app, you must be prepared to offer its full source* to any/all of your customers. Who could then validly compile this source and offer the same product for free.

If you create server-side code which only you host, you are not required to offer source code to your customers.

(*) It's actually a bit more complicated than this. You'll definitely be required to release your source if you statically link GPL code but some approaches to dynamic linking may be exempt. You may have to be prepared to pay legal fees if challenged on this so it's probably easier to just assume you'll need to release all code on request.