Extract the zipped package downloaded from ThemeForest to your desktop, in the extracted package you will find the gullu.zip file which is the WordPress theme.
You can install the theme in two ways:
FTP: Extract gullu.zip file and upload the extracted folder to /wp-content/themes/ folder on your server.
WordPress: Navigate to Appearance -> Add New Themes -> Upload page. Select gullu.zip file. Press the Install Now button to upload and install the theme.
After uploading the theme, you have to activate it. Navigate to Appearance -> Themes page to activate the theme.