After cloning the repo locally there are 2 quick additional steps.
Firstly, you will need the npm command line interace, if you have NodeJs you should already have it. If not, downloading NodeJs should provide you with access to npm commands. (read more here https://npme.npmjs.com/docs/cli/installation.html)
Secondly, you will need to grab the Gatsby command line interface. To do that just run the following command, "npm install -g gatsby-cli" (https://www.gatsbyjs.org/tutorial/part-zero/)
After doing the steps above, simply navigate to the folder in your terminal/command prompt and run "npm install" to install the applications dependancies then "gatsby develop" (which will build the site and serve it at your localhost:8000).