Subject Is it safe to have multiple instances of Gbak with garbage collection?

I believe it is generally safe to have multiple simultaneously running instances of Gbak backing up the same server, when all of them read data from within their own transaction. Is that any different when Gbak is not supplied with the argument to skip garbage collection? Does that put either the backup file or (worse) the database itself at risk of corruption? Perhaps someone can explain how this would work out under the hood.