Only text version of email shows up in Gmail inbox

When I send out a phpList campaign formatted in HTML, in Gmail only the text version of the email appears in the inbox. If I click on the “All” mailboxes link, it shows both the HTML and the text versions of the email sent from the phpList campaign. I would like it to show the HTML version in the inbox by default and preferably not show the text version unless the user has explicitly selected that option.

I am using version 3.2.4 of phpList, version 5.5.29 of php, and I’m having the same issue on any browser/version I use.

The same emails show up correctly with other email providers, in other webmail, on various browsers, but on Gmail only the text version appears in the inbox.

I’ve scoured the forums and Googled it thoroughly, but I haven’t seen anyone else reporting the same issue. I’m stumped. Any help would be greatly appreciated.