Don't know what's up. I don't have that problem with Opera Mail and sending HTML messages to Gmail recipients.
Your recipients could mark those messages as not spam and or add you to their contacts, which should avoid it.
Compose a new HTML messages with "test" in the body that you change the color to orange for example. Send that to a gmail user you know and see if it still gets marked as spam. If not, it might have something to do with the particular formatting you're including in your messages (or a remote image in your signature for example).