Most likely your IMAP-Server/DNS-Settings cause gmail to consider your messages as spam.
F.e. if you don't have an SPF-Record in your DNS for you mail exchanger it may cause this behaviour. There are many possible other reason why it could happen (Insufficient HELO/EHLO host, no mx-record, missing DKIM-signature header, your IP or the IP-range of you ISP is DNS blacklisted, etc., etc.).
Ask recipients to send you the source code of your messages. Most likely the headers contain some informations why the message was considered as spam.