I have an application mobile developed by the flutter and Firestore dB.
I read somewhere that the app must respect the GDPR.
My Firestore content infos user (email, PhoneNumber, image,) and infos for their stores (phones, images, position...)
which data have I to encrypt to respect this GDPR and what also I have to do,
Thanks in advance
You can find most relevant information here.
So it most likely applies to you.
The GDPR is not about encryption, it is about consent. I strongly suggest you read the whole page I linked, there is no way I could explain the GDPR in a single post here.
If in doubt, ask a local lawyer.