Can I create a new table in the internal MySQL within the appliance Gateway?
You can create a table if you wish, but we cannot be responsible for anything that happens to it or it's data.
One thing to be aware of is that the database upgrade (when upgrading the Gateway) might possibly affect the new table, so you should keep backups of the data.
You must also be aware that if an issue arises in the future, then you must be prepared to get rid of this table(s) if we ask. (so that it is not the cause of any issue)