I'm unable to install Garmin Express. Every time I try to open the file "Install Garmin Express.pkg" I get a bunch of text gibberish instead of an installer window.
This is a Mac OS issue rather than a problem with the .pkg file. It sounds like your installer.app is not longer associated as the application to handle the .pkg files.
You will have to reassociate the two. Control + Click on the Install Garmin Express.pkg icon and hover over Open With in the menu. The only option should be the install.app (default). I suggest you Google this to get more in depth or actual Apple Support instructions but if you are familiar with this concept and the file structure you can choose Open With, then navigate to MacintoshHD/System/Library/CoreServices and choose the installer.app.
