Problems with gfortran on Mac El Capitan 10.11.6

I'm an economics student with essentially no computer experience trying to learn R over the summer.

I started working through Heiss' "Using R for Introductory Econometrics", but quickly encountered issues trying to install several of the packages. There was a ton of text, so I didn't read all the error messages, but it seems like the first couple were problems with not being able to find something called gfortran. The ones after seemed to be issues with dependency, which maybe would be resolved if the gfortran issue was.

Looking online, this doesn't seem to be a wholly unique issue, but the explanations I've found so far have been too complex for me. If anyone would be willing to ELI5 to me what to do, it'd be greatly appreciated!!