"In 2 Thessalonians 1:8, why does it say 'obey' not the gospel instead of 'believe' not the gospel? I was always told simply to believe."
Considering that a person's eternal life hinges on it, nothing can be more important than having a biblical understanding of not only what the gospel is but also the situation in which it is being offered.
We often think of the gospel as purely an invitation, like when Paul says in 2 Corinthians 5:20, "...as though God did beseech you by us: we pray you in Christ's stead, be ye reconciled to God." However, God is not merely asking mankind to believe; God has issued a warning of what will happen to all who refuse.
God has "commandeth all men every where to repent [change their mind]: Because He hath appointed a day, in the which He will judge the world in righteousness by that Man whom He hath ordained; whereof He hath given assurance unto all men, in that He hath raised Him from the dead" (Acts 17:30-31).
To consider the gospel simply an invitation would be like saying that God was merely asking Pharoah to let the people of Israel go. We are saved by faith in the gospel, but that gospel is a command from God, and those who refuse to believe will be judged according to their works.
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