Well it’s a lottery, if this IP is clear, next can be on blacklists,…
Google’s anti spam is also driven by user feedback, so you could be able to fix it whit some positive feedback. So I would try to ask your friends and send them e-mail and if it lands into spam, there is “Not spam” button, also you can ask them to mark e-mail as important.
Just do it SLOWLY, no bulk send, preferably make this e-mail unique and in natural language, not just test test or some other nonsense.
Also try to log in into Google Postmaster tool, although it’s useless for small sender never hurts to get another source of information about your mail performance.