Everything you write can leave the app in a standard format, at any time, on any plan.
One note
Open the note menu and choose Export: Markdown keeps the structure for another app, HTML keeps the formatting for the web, PDF is for sending to somebody who just needs to read it. On mobile the export hands off to the share sheet, so it can go straight to mail or files.
The whole library
Open Settings, go to the Data tab and choose Export All Notes. You get a single ZIP of Markdown files with your folder structure recreated as directories, which any other app can read.
Export, backup or sync
Three different jobs. Export produces portable files for the world outside Grape. A backup is a complete archive only Grape can restore. Sync mirrors your library between your own devices. When in doubt: leaving, export. Recovering, backup. Both phones, sync.
Why this matters
The underlying database is a file on your disk. Between that and export, there is no version of this where your writing is stuck inside an app you no longer want to use.