Why don't Gmail users get copies of their own list messages?
If you have a mailing list that includes subscribers with a "gmail.com" address, messages they send to the list won't show up in their own Gmail "Inbox".
This is a problem on the Gmail end. The list server correctly delivers the message to the gmail.com servers, but Gmail discards it. According to Gmail:
Mail you send to a mailing list you subscribe to will only appear in "Sent Mail". Gmail automatically skips your Inbox and archives the message to save you time and prevent clutter. The message will appear in your Inbox if someone responds to it or if there is an error delivering the message. If you'd like to view your message, you can find it in Sent Mail or All Mail.
You can find more information about this on this Gmail help page.
There's no way to avoid this problem, unfortunately. It's something Gmail controls, not the list server.