Cannot enable gtid_mode in parameter group

I'm on MySQL and trying to enable GTID-based replication with an external source in accordance with the following guide: https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/MySQL.Procedural.Importing.External.Repl.html

I'm running MySQL 8.0.26 and according to the guide, GTID-based replication is possible to achieve for this version. However, step 1b of the guide states "Make sure that the parameter group associated with the DB instance has the following parameter settings: gtid_mode – ON, ON_PERMISSIVE, or OFF_PERMISSIVE", and unfortunately it is not possible to edit gtid_mode in the (custom) parameter group I have assigned to the database instance. Any suggestions on how to solve this issue I'm facing?

Best regards,
Max