Which gates should open?
Shabbos 119b: If one answers 'Amen' with all his Ko'ach, gates of Gan Eden are opened for him [to enter]. We read "Pischu She'arim v'Yavo Goy Tzadik Shomer Emunim" like 'she'Omrim Amen.'
Rashi: The gates of Yerushalayim. Malbim - the verse calls to the gatekeepers to open them. Until now, they were closed in front of them for two reasons. (a) The 10 tribes served idolatry, and were separated from Anshei Yehudah. Now, they are "Goy Tzadik"! (b) Refer to 26:2:2:5.
Radak: This is a metaphor that Eretz Yisrael should open for them; now, it is as if it is closed in front of them.
Why does it say "Shomer Emunim"?
Sanhedrin 110b: A baby has a share in the world to come from the time that he says 'Amen' - "...Va'Yavo Goy Tzadik Shomer Emunim" - we read this 'she'Omer Amen.'
Refer to 26:2:1:1.
Rashi: The Goy Tzadik waited in Galus and for many days anticipated the Emunah of Hashem to fulfill His promise via the Nevi'im to redeem them. "Shomer" is waiting, like "v'Aviv Shamar Es ha'Davar" (Bereishis 37:11), "v'Shamar Hashem Elokecha Lecha" (Devarim 7:12).
Radak: In Galus, they guarded Emunah in Kel, and did not do idolatry - "Im Shachachnu Shem Elokeinu va'Nifros Kapeinu l'El Zar" (Tehilim 44:21).
Malbim: The 10 tribes rebelled against Malchus Beis David from the days of Yaravam and onwards. Now, they are Shomer Emunim - they will not divide into two kingdoms again.
