Yes, you can install the PHP extension on App Platform.
When using PHP apps in App Platform we recommend using Composer to configure them. This would allow you to specify GD Library in your composer.json file and have it install.
If composer is set to include the GD option then it should pull the image with it enabled. If composer isn’t an option you can use a Dockerfile instead and have it install GD library during the build process.
To install php GD you would need to use Composer with your app to specify that version.
Something like this should work:
That would be configured in your composer.json file. As we use Heroku build packs typically composer is the recommend route for anything PHP.
You cannot use the App console for this. The local filesystem on the host instances running App Platform containers is ephemeral. When deployments happen or containers are replaced the local disk storage will be LOST and not recoverable.
Hope this helps!