There are a number of reasons that Gmail may reject your email messages, but by far the most common reason happening right now is a problem with your domain's SPF record. Gmail has started to flag many email messages as spam when there are problems with your domain's SPF record.
What's really strange about this issue is that some emails make it through and others do not. A common delivery error message looks like this:
email@example.com: host gmail-smtp-in.l.google.com[xxx.xxx.xxx.xxx] said: 550-5.7.1 [xxx.xxx.xxxx.xxx] Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. Please visit 550-5.7.1 https://support.google.com/mail/?p=UnsolicitedMessageError 550 5.7.1 for more information.
It could be that your SPF record does not include all the IPs your domain sends from or your domain does not have an SPF record at all. Not having an SPF record at all was not a huge problem for delivering to Gmail until very recently.
If you are using SpamHero to send outbound email, see the following article in regards to fixing your SPF record: