Hi GamePad64 welcome to the forum
This is two different questions.
The CLA documents that a contributor agrees to let Discourse use their code.
The GNU GENERAL PUBLIC LICENSE is what makes Discourse open source.
AFAIK there would be no issue with your using the Discourse CLA as a base for yours.
I take it your main concern is the conflict between GPL and the app store’s DRM policy.
IANAL (talk to one) but my take is that if your code is using any Discourse code, the GPL will apply and you could have problems. If on the other hand your code is entirely your own you can license it as you see fit.