My email always going into gmail SPAM

There is two main reasons:

1. You are missing rDNS, SPF or DKIM or all three, plus it would help if you have DMARC.
2. Willingly or not your IP and/or domain is blacklisted with Gmail.

First one it should be easy to configure if you have "real" server. But if you are hosting from home your ISP will never set rDNS and because of that your problem can hardly be solved.

Second option is the worst as Gmail is not easy to forgive and for any serious spam they will blacklist your IP from 1-2 months up to more than a year without any chance to remove yourself. When i say spam that doesnt mean you need to send thousands of emails, its enough just few without proper configuration and the chance to be blacklisted is here. So dont take it easy when your emails are redirected in spam folders.

Now what is going on is hard to say without log files but by my experience when it comes to Gmail this are the main reasons why they could reject your email or mark it as spam.