First, unzip the file you download to a directory on your hard-drive you remember. Then go to Google Chrome and open the Settings menu in the top-right, choose “More Tools > Extensions.” With the Extensions panel open you will find a Developer Mode checkbox, probably on the top-right corner; please enable it. Now you will see a new button called "Load unpacked" or "Load unpacked extension," please click on it and then choose the Gravit Designer folder that you unzipped earlier. Now you will see Gravit Designer in the list of extensions.
You can also install it from the Google Web Store.