Gist vs. Main Idea
The gist is your initial thinking about what the text is mostly about. The main idea is the key point the author wants you to take away from reading the text, and is found by careful analysis of the text for details. Sometimes our initial thinking about the text (the gist) is the main idea, but this is only confirmed through careful analysis of the text and finding details to support it.
See this in action and hear this explained by fifth graders in Detroit Public Schools, as they analyze Article 2 of the Universal Declaration of Human Rights, as part of the EL Education Language Arts Curriculum.