All I know about Gary Johnson is that he's a former governor of New Mexico and he's the presumptive Libertarian Party nominee. There was a profile on him recently in the New York Times (
http://www.nytimes.com/2016/04/06/us/po ... arian.html)
One of the interesting things about this election cycle is its providing a rare opportunity for smaller parties, like the Green Party and the Libertarian Party, because so many people are dissatisfied with the Republican and Democratic choices they're seriously considering a third party. Johnson has been doing reasonably well for a third party candidate (he also ran in 2012, and according to the NY Times he got around a million votes and came in with 11% of the vote in a recent poll) and its possible, though not probable, that he might qualify for the presidential debates in the general election (which is usually based on averaged poll numbers).
Personally I don't vote Libertarian because I think regulations are too important, but I have a tremendous amount of respect for Libertarians. Theirs is a very principled political party--and I can understand their argument even if I don't agree with it.