Contradiction with Matthew 28:19
Isaiah 27:12 implies gathering from one specific area, whereas Matthew 28:19 speaks of spreading the message globally.
Matthew 28:19: Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: [teach...: or, make disciples, or, Christians of all nations]
Contradiction with John 18:36
Isaiah 27:12 suggests a physical gathering on earth, while John 18:36 indicates that Jesus's kingdom is not of this world.
John 18:36: Jesus answered, My kingdom is not of this world: if my kingdom were of this world, then would my servants fight, that I should not be delivered to the Jews: but now is my kingdom not from hence.
Contradiction with Revelation 7:9
Isaiah 27:12 focuses on a specific population, while Revelation 7:9 describes a multitude from all nations and tongues.
Revelation 7:9: After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands;
Contradiction with Acts 10:34-35
Isaiah 27:12 presents a specific people being gathered, but Acts 10:34-35 reveals God's impartiality and acceptance of people from every nation.
Acts 10:34-35: Then Peter opened [his] mouth, and said, Of a truth I perceive that God is no respecter of persons: