Google uses pretty sophisticated spam filters, so there is going to be a lot more automation and detection of spammy content. Desktop email clients like Outlook are typically going to rely on user created filters, so wouldn't pick up on as much.
The most common reasons you might be showing up as spam are your mail headers, your mail server's IP sending reputation, and the content of your message. I'd type up a more in-depth explanation, but it looks like there is this older phpList forum post on What can you do to avoid being marked as spam
that would cover most of the same stuff.
Also, you might try reviewing straight from Google's own guidelines on Why messages are marked as Spam