Galatians 1-2

How can there be 'another gospel'? From where did Paul get the gospel?

There can’t be another gospel. Paul received the gospel by revelation from Jesus Christ (1:11)

If justification comes through works of the law what did Christ die for? What is nullified if justification comes through works of the law?

If justification could come through works of the law then Jesus died for nothing, and the grace of God is nullified.