-l Make bash act as if it had been invoked as a login shell (see INVOCATION below).
--login Equivalent to -l.
This is an excerpt from the bash manpage. So likely no, this isn't a bad thing and this is just GNOME sourcing your environment variables. You need to tell us how frequent these notification are and when they happen (also what GNOME version you're using as well as which distribution you are using). We can't help you if you don't provide that.
Then likely you can ignore it, or configure GNOME notifications to not display it to you. You could also seek help in NixOS related forums or continue trying to replicate the issue.