on a mac air notebook I have no trouble opening other tgz downloads from GPS file depot. Today I downloaded and opened North and South Dakota maps in that file format. However,MT Topo + National Forests 2016.gmapi-87.tgz ....... I have downloaded it three times. I have changed extension to .zip. I have tried using UNIX commands in terminal mode. None of the should be necessary. I suspect the file is corrupted.