Would it be possible to use the existing database on a new gigalixir project and what would the costs be?
We are actually providing you with a database instance, not a database. A database instance can house multiple databases. The connection string for your instance will look something like this postgresql://gigalixir_admin:PASSWORD@11.22.33.44:5432/DATABASENAME
The gigalixir_admin account is an admin on the instance and you can create a new database with standard Postgres syntax (eg. CREATE DATABASE …). Just make sure you understand that you are sharing CPU/memory/bandwidth/connection count between all clients (apps). Also consider that the database will probably get a little restrictive as your apps grow.