The way I understand it, gist goes with the sub-skill of skimming...reading a text quickly just to get the general meaning. Scanning is looking for something specific in the text. Intensive or detail reading is for reading for a deeper understanding of a text. That's what I've got in my notes anyway. =)
My understanding is that gist reading or listening is done to get the "gist" or basic idea of the text/recording. Skimming is listening or reading with the intent to get the gist. Scanning is looking for specific information. We scan a phone book for a specific name and number. We skim a newspaper article, sometimes only the first paragraph or two, to get the gist of the article. Then if we want to read in more detail we can. Would like to know if that's what others think too, though.