I wondering why Catholics Pope suddenly make an u-turn on gay marriage?, aren't this was against the Bible teaching?

It’s essential to note that the document doesn’t change the Catholic Church’s teaching on gay marriage. It’s prohibited — without exception.
What is evolving is how the Church responds to LGBT people who participate in same-sex relationships.

This Christmas season, priests as usual will visit prisons across the world to give their blessings to rapists, pedophiles, and murderers. If they can do that with people whose sins are nearly unforgivable, they most certainly do that with people whose “irregular relationships” are not.

A blessing isn’t an endorsement, it is not an approval.
It’s an invitation to God and the persons receiving it to grow closer to God.
This unconditional love, preached by Jesus Christ, forms the bedrock of Christian ethics.