What gate of heaven will "Christians" enter if there are only 12 gates with each of the names of the 12 tribes of Israel?

Baptized Christians will be able to enter at the gate of their choice.

A couple of more gates are needed, Ephraim and Manasseh.

It is a figurative description and really not all that significant as your question poses.

They are already there through Jesus Christ the Door. We all wait for the rebellious Jews.

I am the door: by me if any man enter in, he shall be saved, and shall go in and out, and find pasture. 

You've officially put way more thought into it than any Christian in history.

The Bible says that in heavenly Jerusalem, the names of those to inhabit it are already inscribed there. Hebrews 12:18-24 and Revelation 20 adds that it's whether your name is already written on the Lamb's Book of Life on the Day of Judgement that determines whether you then get into Heaven. As for those 12 gates, Revelation 21 says that written on them are the names of the 12 tribes of Israel and on the 12 foundations are written the names of the twelve apostles of the Lamb.

There is a 13th gate which everyone who does not belong to a tribe can enter e.g. converts.

Not sure, never heard this one before.  Christians would say that only the chosen go through this ordeal at the end of days.  They will say that Christians will already be there since everything changed with Jesus, and they will already be on the other side.  They got in the back entrance.

Warning = this is a troll with multiple accounts.

Just block and ignore him, as such don't care for answers.

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None of what you speak of is real, only the natural world exists.